How do you properly check servos out? And what equipment do you need ?

Here is what I do, Russell.

Better if servo is out of plane, but will work with servo mounted also.

While slowly making servo move from extreme throw to opposite throw, carefull listen and feel for smooth operation. If it feels/sounds notchy or grindy, check the gears for a stripped gear or a few teeth broke off.

Also with radio turned off, manualy move the output arm slowly back and forth and you can feel any stripped gear teeth as they try to mesh or not.

Apply some resistance to output arm with finger while you move the transmitter stick for that servo back and forth. You want to feel the proper amount of torque in both ways. Real weak servo could be bad motor.

You can perform any of the above tests with a new/known good servo in order to be able to do proper comparison to the servos being tested.

Some more checking would involve tapping on servo while operating it and wiggleing/pulling on servo wire pigtail to see if it might be vibration sensitive or the wire going into servo might be damaged.

Really rare for a servo that has been in normal operation for some time to fail, but can happen of course.
I usually perform the test to a new servo before installation and to servos that have been in bad crash.