Private investigators have come a long way since Philip Marlowe pounded the mean streets. Not only can you now train online (thanks to the Institute of Professional Investigators), the internet has made it possible to crack cases without moving from your computer. Anyone who knows where to look and what they are looking for can be a cybersleuth.

How to become a private eye

Anyone can be a private detective. You could get yourself an old mac and a notebook and start tomorrow without so much as having read Raymond Chandler. But not for much longer. The Security Industry Authority (www.the-sia.org.uk/home), a government quango, is trying to stamp out the trilby and dodgy mac brigade and, as soon as it decides how best to do this, it will license private detectives. In the meantime its site has a useful guide on “best practise for private investigators” (even if practice is spelt wrongly).

The SIA recognises the Association of British Investigators (www.theabi.org.uk) and the Institute of Professional Investigators (www.ipi.org.uk), which both support licensing and recommend training. The ABI has a downloadable file on how to become a private investigator, and links to courses on the likes of surveillance and tracing elusive debtors.

The IPI has just launched its own online distance learning package (£169) and optional exam. Passing this won’t give you any special powers, but as Nicola Amsel, the IPI principal, says, the main difference between private investigators like her and everyone else is “we know who to ask, how to ask, where to look and how to collate the information”.

Do your own sleuthing

You don’t have to do any special training to be a cybersleuth. Do a little virtual curtain-twitching and you can build up a profile of someone with remarkable ease online. If you are trying to track a missing relative, Peter Heims, a private investigator for 53 years, and spokesman for the ABI, says start with Google, then go to www.192.com. Put in the surname and town and you might get somebody’s address, phone number, how much they paid for their house, who they live with, and, if they happen to be a director, their date of birth, nationality, occupation and company name. You can get the financial lowdown on the company at www.direct.companieshouse.gov.uk. For a small fee you can visit the Land Registry (tinyurl.com/japbs ) to find out if someone has a mortgage on a property or get birth, marriage and death certificates at www.gro.gov.uk .

As the BBC family history site explains at tinyurl.com/2q6k5d, these documents can be dynamite: a marriage certificate, for example, gives the date and place of the marriage, the age, occupation, marital status, profession of both parties, the name and profession of the father of both parties, and the names of witness. It will also give the parties’ addresses.

Don’t stop there. Bankruptcy orders are available at www.gazettes-online.co.uk and if you want to embarrass someone, is it your fault if they put up pictures saying “Just do me any way you want” at myspace.com?

Have-a-go hero

A citizen’s arrest sounds like a fine thing, but it is a brave man who does it. As the BBC Action Network (tinyurl.com/2my3xc) points out, if you make an arrest using “reasonable force” but the arrest is unlawful you can expect to end up in the dock yourself. The site links to the K-Zone, tinyurl.com/2owg6o, which explains this minefield in more detail. For example, failing to give details to another driver at the scene of an accident is an offence, but it is not an arrestable offence, so don’t imagine you can take their car keys to stop them getting away. This would be unlawful arrest and the courts are unsympathetic to private individuals who get it wrong.

The case of Tony Martin, the poster boy for the have-a-go hero lobby, who shot and killed a burglar, is discussed at neighbourhoodwatch.net, and further serves to underline the complexity of the law. For example, while anyone can use “reasonable force” to protect themselves, if you chase an intruder you are no longer acting in self-defence and the same degree of force may not be reasonable.

Joining the Old Bill is safer. If you can spare a minimum of four hours a week you could become a special constable, says policecouldyou.co.uk . “Specials” are part of an unpaid but trained force of volunteers who support the police’s work, especially in terms of public disorder.

Sleuthing gadgets

Wannabe Sam Spades can sleuth in high-tech style, but the law spoils much of the fun. In case you have been on Mars, telephone intercepting is illegal, and much of what is sold by the likes of active-spy-shop.co.uk will have the law snooping on you. Certainly the shop didn’t want to speak to us about what it sells, and a lot of so-called spy shops are curiously reluctant to answer their phones.

The law is full of curious quirks. Planting hidden bugs in a room or office isn’t illegal, according to the informative www.out-law.com (part of international law firm Pinsent Masons), provided you are in the room or office legally. It is how you use the data collected that may interest the police.

At the Practical PC site (tinyurl.com/2jvhr8) there is a guide on how to use Windows XP Professional’s encrypting file system , which lets you encrypt individual files or folders on a computer, share them and transport securely, and www.onlinespyshop.co.uk sells military-grade encryption software to secure mobile calls and texts between two handsets for £599.

Cover your tracks

You don’t have to have your personal details made available to the world if you don’t want to. The FAQs in the help and support section of 192.com describe how to get your privacy back. For example, you can download a form to stop your electoral roll details being used and for a £10 fee registered users can also get details on who has searched your details.

But the telephone records at 192.com come from a live feed updated daily. You will have to contact your service provider to get your phone number deleted.

To stop cybersnoops spying on your online activities, you can hide your IP address with Anonymous Surfing. Download a free trial at www.anonymizer.com or get a year’s subscription for £15. If you have had personal documents stolen, register at the fraud prevention service CIFAS, tinyurl.com/26x2cz, to stop them being used fraudulently. It costs £11.75. And if you are really serious about your privacy, get yourself a shredder. A Which? best buy, the Fellowes Shredmate shredder, is £24.99 from www.argos.co.uk .