Is this a safe way to test an isolation transformer?

[Sectorseven]

Plug it in and put a voltmeter on the other side?

[WhereEaglesDare]

Sure, depending on the rating of your voltmeter.  What are you using the isolation transformer for?

[Sectorseven]

For a WG monitor, but the label is worn and I wasn't sure what voltage it's supposed to produce, or if it even works at all.

[qrz]

an ideal isolation xformer produces the same voltage on the secondary as is applied to the primary
due to a 1:1 turn ratio

due to various losses , its not quite 100% in reality . but, close enough 

[MonMotha]

While you're at it, you should also check that the transformer is, in fact, isolating.  This can be done without power applied.  Using the continuity or resistance setting, check each output terminal to each input terminal.  Should read high resistance (no continuity).  The two output or two input terminals will read low ohms between each other, but you don't need to check for that if you're verifying operation by checking in/out voltage with power applied.

[qrz]

just don't hang on to either set of windings and drop the meter leads .
the collapsing field will induce a voltage that CAN tickle a bit - i.e surprise !!