get one of those fancy servo testers. run new servos through it record the performance. then over time recheck your servos and if see a change in performance then change it out. Do this servo check with a load on it. use a quick clamp in a mount to hold the servo. put on a large 2" horn and hook it up to a spring, R/C car shock light weight one, rubber bands or what ever to add a load to it.

like i said you can see the performance of the servo and over time if bad change it. this should help find a bad servo if new also.