You can invest in any one of a number of electronic servo testers made for the hobby. They start at $25 but one I saw
recently at a friends shop was the Servo Exciter for about $75. It tests quite a number of functions and seems to be a worthwhile piece of gear especially if it willidentitfy a faulty servo BEFORE you install it. I had a bad servo in my Raptor 60 at first but didn't know it as it would intermittantly tilt backwards in a hover. I made many inspections and changes before I discovered the servo was bad. Had I used a servo checking device before installing that new "bad" servo I would have resolved the problem easily. It could have caused a serious crash. I was lucky.