Johny B Good submitted this idea :
I keep lots of cheap ones around and some some very expensive professional ones. I've never bothered to count, but at least 25+. I would not recommend a cheap one for mains voltage and current levels, because they are poorly built and down to a price. The risk is of them suffering an internal fault and with several hundred amps of fault current behind them, they have been known to kill. There are several quite graphic examples to be found on the net, complete with photos.
Again no. They just cannot be relied upon. They are dim, they need the user to be earthed and if they suffer an internal short, you get a belt or worse.
For mains work, a Fluke voltstick is far more useful and much safer. You don't need to be earthed, the wires do not need to be bared, or have any metal to metal contact, because they pickup the field around the cable.
With practise, you can even trace the live wire in a 3 core flex and get a fair indication of whether the neutral might be missing. You can even check whether a cartridge fuse has blown in a dis-board, without disturbing any of the other circuits.
Best thing since sliced bread, in the hands of someone experienced - but they still need to be backed up by the use of proper test equipment, before working on a circuit, to prove it is isolated.
They are only good to prove something is live, not to be relied upon as proof that something is safely isolated.
You can test a volt stick is working, by rubbing it's nose on your hairy arm.