A quick way to check would be the noise it makes under load. Try plugging it in, leave it at neutral and try to push the servo horn to one side. Digital servos have a tendancy to whine and buzz quite loudly in this situation.
It's not foolproof but it'll give you an idea
Another thing to check is overshoot returning to neutral. If you hold the stick over one way and let go an older analogue servo will go slightly past centre on return to neutral.