I give them a good visual inspection...then turn the output arm carefully by hand, (no power ) and listen/feel for potential gear damage...
Then plug 'em in and test for unusual noises/movement...using a servo tester.

I have a beat-up old trainer that I keep around just for testing old or used equipment..radios, engines, etc.


Edited to add: "using a servo tester" as mentioned by others in previous posts.