a quick visual guide to coil stops being bad is a noticeable amount of gold or silver colored dust at the bottom of your cabinet underneath your flippers. this indicates that the coil stops are being chipped by the impact of the metal plungers.
as previously stated, the quality of the parts being used today is noticeably less than 5-10 years ago.
even if you do not notice the metal shavings, it is a good idea to remove the screws holding the stops in place and visually inspect them. mushroomed coil stop ends can impact flipper performance as well dirty, swollened, cracked and/or broken coil sleeves.
coils that have overheated can also impact the plunger's ability to move smoothly though the sleeve as the coil is energized. if the coil sleeve is not able to be removed easily, that can indicate either a damaged sleeve and/or coil. coils where the inner plastic liner has expanded will not allow the plunger to slide as easily as it should. depending on the severity, you can run a drill bit through the coil and open up the inside again so that the coil sleeve fits easily and the plunger travels without any resistance.
coils that overheated to the point where the plunger and/or coil sleeves are trapped to the point where you cannot manually remove them, should be replaced.