A Servo tester and an amp meter allows you to test the servos. A digital position readout is a useful addition.
With the one I have, 1000 1500 2000 are the three prime positions of concern.
1500 is obviously 50%, and is very useful when you are setting linkages, etc.
Once a model is trimmed, checking the servo positions can also be useful if you want to eventually fine tune
linkage, etc, or transfer to another TX.