Mains socket tester on sale at Aldi
Aldi are selling a mains electricity socket tester as one of their weekly deals. These useful little units will show if your electric sockets are wired correctly. I have used one in the past to prove there was a problem with a dodgy earthing connection affecting the whole building.
They come as a simple plug in unit with three lights. When plugged into a mains socket different combinations of lamps light up depending on whether everything id OK or if not what exactly the problem is. To make using it easy the plug has the various lamp states and what they mean printed on it.