I replace coils when they are the wrong value (and this happens a lot), or if the coil sleeve is melted to the coil. Simple test is to measure the resistance and if it is out of tolerance, then I throw it in the bin. Sometimes, you can replace the coil diodes. If the coil looks UGLY, then use the coil wrappers from Peter's site ;

http://www.pinballrebel.com/pinball/cards/