Has your wife recently started putting her make-up on before taking the dog for a walk? Does the dog-walking take longer than it used to? And does she come back looking a lot more knackered than the dog?

Well, at least you know. You know that your wife is having an affair with the man with the german shepherd. You’re not dumb, you’ve been totting up the telltale signs for ages: the frequent out-of-town conferences; the receipts for a day room at Heston services; that Christmas drink with Cheryl last week (except you happened to see Cheryl having a Christmas drink with someone else); the german shepherd hiding in your wardrobe.

But what about those poor husbands and boyfriends who nervously suspect but only know for certain when they get home from the pub one afternoon and find a note on the kitchen table saying, “I’m moving to Malaga with Geoff”?

Until now there were only two rather unattractive courses of action to confirm marital infidelity. You could confront your beloved or you could hire a private detective. Confrontation is lose-lose: if he/she isn’t cheating on you, you’re going to cause unimaginable trouble by suggesting he/she is. And if you were right all along, it’s nothing to be inordinately pleased about, either.

The other option, a private detective, certainly gets the job done – you only need to ask Ingrid Tarrant, who successfully hired a sleuth to confirm her suspicions about husband Chris’s after-hours relationship with Fiona McKechnie, a blonde primary school teacher, but for the rest of us who are not celebrities, the problem with hiring an investigator (apart from the dubious ethical ramifications of having your wife followed by a man in a mac with its lapels turned up) is the money.

PIs cost anything from £40 to more than £100 an hour and usually work in teams of two. Which means you’re shelling out up to £2,000 a day. Not very credit crunch. If your beloved’s assignations are infrequent, you could go bankrupt long before you know the truth. Then they’re going to leave you anyway.

Now, though, there is a third way: you can spy on your philandering wife (or, I suppose, husband) yourself. It’s no longer an unsophisticated matter of rifling sweatily through her handbag or underwear drawer, or checking up on her during her lunch break. No: the market in spyware has recently exploded with a level of gadgetry more at home in James Bond’s glove box now legally available to everyone. Call it a late Christmas present. Or, if you’re lucky, a sales bargain.

My assignment last week was to test the latest “over-the-counter” spy gadgetry on my own beloved wife. I agreed – largely because I am contractually obliged to do so; but also, for a few weeks now, there have been some unexplained . . . absences. But, as I said, it was most definitely all in the name of work. Not because I’m suspicious. Honest.

I could immediately see from the range of equipment available that, armed and ready, only the most inept prying husbands would let their quarry get away. From pens with cameras to snooping widgets that tap into mobile phones, there were devices that could do almost everything.

Among the more flamboyant options were the “spy kite” – a not terribly subtle craft that can fly to a height of up to 80ft, usually employed to peer over garden fences; a brooch with an inbuilt video recorder; an air freshener aero-sol that captures images and sound; a “nanny cam” – a Winnie-the-Pooh alarm clock that transmits CCTV; and, possibly the most ingenious, a camera disguised as a tin opener – you know, just in case there’s any untoward kitchen sex going on. I decided to overlook the spying websites that offered guides on how to assemble your own stun gun or throwing net. Subtlety, after all, is essential to DIY spying.

Phase one: bugging the house. In the old days a bug worked using radio waves. Meaning it had a range. Meaning you had to stand within 50 yards of the room you were bugging, wearing a large pair of headphones and a suspicious expression to hear anything. Now the latest bug uses GSM technology, so you can phone it from anywhere in the world and listen in without contravening any communications laws. The bug I borrowed was disguised as a 13-amp multi-plug and costs £570. But through Spycatcher Of Knightsbridge you can buy a less high-tech bug for £161 that can be hidden in a wall clock, a spectacle case, a picture frame or, if you’re really cruel, a basket of pot pourri.

Once it’s planted, you dial its mobile phone number and suddenly, terribly, deliciously, you’re in the room. It’s like talking to someone on hands-free, except you’re not talking – you’re listening. Or, if we’re being honest, snooping.

As I snooped on Harriet chatting away to a couple of friends, I felt as though I were breaking some ancient marital vow. Which I was. If I truly were a suspicious husband, I might have carried on listening all day – but I’m not and I got bored. My goodness, women can natter.

Phase two: snooping on e-mails. When I got home from the office that night, I crept over to Harriet’s computer, typed in a code – and up pinged a text file containing every keystroke she had made that day. Girlie e-mails, passwords, Google searches, online shopping details. I know, you’re right: this stuff is terrifying.

All I had done was to place a tiny widget (£167) between the keyboard lead and the socket on the computer that morning. The widget records everything typed and reveals itself on the screen only when you enter a password. Because I like being married and because my wife will be reading this, I told her what I’d done before looking at the file. Understandably, she got stroppy and demanded to see it first, then deleted large swathes of it. What was left didn’t make a lot of sense.

The widget records everything, so it’s not brilliant if your cheating wife can’t type particularly well – which may be even worse if she is in the advanced stages of passion. For example: “]I didn’ mit tone th heb resnt ie, bt I thi we shold il do it – astlady atth ln said she woldb need another oebeore the edding.” But you can search for key words hidden among the gobbledy-gook. Such as: “I want you”, “Geoff”, “room 43, 8.30pm”, and “yesterday lunchtime was amazing”. I found none of those. Maybe because of the deleting.

Phase three: the creepiest bit of kit so far – an innocent-looking Sony clock radio with a state-of-the-art colour video camera and microphone hidden inside. Bedcam.

“Darling, I’ve bought us a new bedside clock. Because I know you found the old one a bit . . . old.”

“Oh, how sweet.” It costs £1,145 but at least you don’t need to stand in the park outside wearing that mac. You simply dial it from a video phone anywhere in the world and an image of the room pops up. If you wanted to, you could monitor bedroom activity while you were attending that conference in Wolverhampton.

I am delighted to reveal that I learnt nothing scandalous from the video radio. Mainly because I didn’t have a flash enough video phone. Nor did I glean anything illicit from the video pen (yes, a pen with a video camera in the top – £387). Nor from the voice-recording pen (£177).

I gave the night-vision monocular (£240) and baseball cap body cams (£632) a miss. But with the car tracker (£2,129) I had discovered what I’d been looking for.

Phase four: the car tracker (because you can bug your house as much as you like, but it’s highly likely that the cheating will be taking place elsewhere). The tracker follows the car anywhere in the world and you can programme it to alert you if it travels a certain distance from home.

If you think you know where Geoff lives, you can ask the tracker to tell you if it gets to within 250 yards of his address. You can’t ask it to disable the car if it does, but that level of technology is surely just around the corner.

Monday morning, bright and early, I stepped out onto the drive. Checking the road to make sure nobody was spying on me, I slipped under the car and attached the device behind the bumper. I fired up the accompanying mini-computer – and there beeping on the screen, as if I were in The Bourne Supremacy, was our Skoda estate (the perfect car, I’m sure you’ll agree, in which to conduct a steamy romance).

Off I went to work. Off Harriet went “to the swimming pool, a bit of sales shopping, then to see Louise”. That’s what she said she was doing. For one hour and 20 minutes, our car was parked in some residential street nowhere near the swimming pool or any shops or, for that matter, Louise. I knew that street. I knew who was there.

“Good day, darling?” “Yes, you?” “Fine. What did you do?” “Swimming. Blah, blah. Shops. Got 70% off some trousers. Blah. Louise. Blah.”

“Nothing else?” “No” – but I saw her eyes flick away. She was lying.

“Are you sure?” “Yes.” “Then why was our car parked outside the spa place for an hour and a half this afternoon?”

“I’m sorry?” “You had a massage. J’accuse!” She flushed red. And then she admitted it. She’d been seeing a reiki healer each week. Hurt, I asked how long this had been going on. She refused to answer. Instead, she told me that if I didn’t send all my pathetic spying stuff back to the shop immediately, she would run off with someone. And they wouldn’t just be good at reiki.

Even if I had discovered something untoward, I’d have difficulties using my material in the divorce courts. “I would always advise my clients thinking about using underhand ways not to do it,” says Marilyn Stowe of Stowe Family Law.

“It almost always rebounds and you could end up liable to be sued yourself. Judges don’t like this kind of evidence and often it doesn’t work.

“Emotions run high during divorce proceedings and sometimes people who would never normally dream of employing such methods wonder if they should resort to using them. If a client produced evidence that had been obtained in an underhand way, even if it was damning, you’d probably find that the jury would think badly of the person who went to these lengths.

“I heard of a case where a wife got an agent to download the contents of her husband’s computer. Her husband ended up successfully suing her and her lawyers for interception of data. You expose yourself to far greater risks than the outcome could justify – it’s not worth it.”

That’s a warning to the curious if ever I heard one. I think I’ll leave spying to others in future.

All equipment from www.spycatcheronline.co.uk