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.
Now that Gene is long out of business, any idea where you can buy the f-14 tomcat blue and red chevron inserts?