Most of the issues are pretty obvious by just moving the mechanism by hand. If the glass if off for any reason, I'll usually check the flippers almost unconsciously.
Just grab the flipper and wiggle it sideways, that'll instantly reveal any play in the bushing.
Worn out link can be tested by stopping the plunger with a finger and then wiggling the flipper bat among its normal path. Any play will be very visible when looking at it from below.
For most problems I don't have to open the machine, I can just feel it when playing and diagnosing by playing is how I catch most of these cases.
Space station has high current end of stroke switch, which needs to be filed clean every now and then. There's always a bit of arcing at the moment when it opens and since the high power side of the coil gets its power over that, issues with the switch will make the whole thing feel tired. Flipper button switches are another place that needs periodic filing.
That and replacing the wear parts should restore their strength.