Multi-voltage power supply
There are two ways to do this check; one, of course, is to examine the
power supply for a label.  Most AC-input power supplies are well-labeled so that
the user does not need to guess how to safely plug them in.   It may
be necessary to examine the very fine print, closely. It is sometimes
necessary to ensure that a switch or jumper connection is made, on
some older power supplies, to select a voltage range.

The other way, is to reverse-engineer the power supply, by examining the
internal circuitry.   Sometimes the power connector is suitable only for one
national standard power connection, and examining the plug is a very
strong hint.   

Checking the output power is easy; just apply a voltmeter while a
suitable load (usually a resistor) is attached.   One would use the
voltmeter, also, before touching any metal on an unlabeled box that
accepts AC power, to ensure you don't get a shock.