Check that there is continuity of the Earth with the metal enclosure.
Measure with a ohm meter that the live and the neutral is not shorted [depending on the watts of the equipment, say above 10 ohms]. Make sure you switch the unit to the On side while measuring the resistance.
To be on the sure side, use a incandescent lamp in series to turn the unit on. If there is no short circuit, then you can plug the power direct without the lamp, after that.
Also use a isolating transformer if you have one. Not a variac.
If you can, open up the unit for a casual visual inspection if anything is missing or obvious hazardous situation that do not justify it to be plug, example missing circuit board.. etc...
Hope it provides some insights to you.