Cables should be installed in 'safe zones', but this does not mean they are!

Safe zones being vertically above/below sockets/switches, horizontally left/right of sockets/switches, 150mm from corner of walls/ceiling.

You can get cheap detectors, but they are hit and miss.

More expensive "cats" can be hired, and look for either metalic objects, AC waveform, or an injected signal tone on dead cables.

Google ezi-cat.

I have a cheap cable finder/stud locator that I got from Focus. It's quite good at detecting the voltage from a powered cable but for anything else it can be a fiddle to adjust the sensitivity - you have to set it so that it's within a wisker of triggering but not actually (false) triggering. There is definitly a knack to this. Once you get used to this it's ok for the price but this difficulty might account for some bad reviews of similar cheap products. If it's just one job you want it for I'd try a cheap one from somewhere like Screwfix which will normally let you take something back if you can't get it to work.