I would turn the unplugged servo in both directions...check for roughness, weird noises, etc. Go to both extremes.

Then plug it in, and test it with the radio....check for noises, movement to end limits....grab it..test it to see if it will take a load.

Not very scientific I know....typically they work or they don't. It's pretty obvious if it's stripped. In the past I've had servos "chatter" after centering the stick...kind of like they're cycling. When they do this...I just toss them. Standard S3004s are about $16...not worth it. Besides, it's pretty uncommon for that to happen anyway.