Anybody who builds robots for a living knows that not all servos work perfectly out of the box. If you know your electronics, you can always build your own servo tester. The CircuitPython Servo Tester from Adafruit can test the range of motion of a servo (0-180 degrees) and test the smoothness of the motion.
You are going to need an Adafruit Feather M0 Express, FeatherWing OLED, and a few other simple parts to pull this off. More info is available here.