Quoted from vid1900:
Light up inserts from the front with a flashlight and check each one for damage. You want to look from the back because there will be less to distract your eye.
You can sometimes reinforce a broken insert with epoxy and chopped glass fibers, but if you are putting a lot of work into a machine, you probably want to order new inserts from Gene at Illinois Pinball.
Illinois Pinball doesn't have inserts on their website...?