Here’s a pretty good servo tester, it’s pretty hardcore –
The Servo Tester 2 is hardware and software that, basically, makes a pulse width signal between 1 and 2 ms. This is the signal used to control common R/C servos. There are numerous servo tester construction articles on the Internet. This one has some advanced features, not usually found on servo testers.
Servo Tester 2 – Link.