The hardcore test, still "safe". Only to be used if really needed.
It assumes that the voltage is standard 11V / 220V.
Use an thin wire, properly insulated at one end, NOT insulated at the other. Some screwdriver should do the job just fine.
Be VERY CAREFUL not to touch an metal part of the wire.
Insert the wire into one of the holes of the socket.
There's no completely foolproof way to do this.
You can plug in a lamp or other appliance, but this is prone to false negatives (situations where the appliance won't work, but the outlet still has electricity). The most common situation would be a broken neutral wire (the hot wire is still live, but with nowhere for the electricity to go, the appliance won'...