Checkin Mag heads for cracks

Take a wire brush on a drill and clean them right between each valve.
Clean them off with brake clean right between the valves.
Let everything dry.
Visually inspect them.
Hit them again using a straw on the brake clean right between the valves, just a little brake clean not a lot.
If they are cracked usually the crack will be the last place the brake clean evaporates from.
This is about the cheapest way I know without paying the shop to inspect them