First learned to use a piece of wax paper and then 2 or 3 magnets of varying strength. Start by checking the places you'd expect to find filler used by hacks. (rockers, blower doors, around windows, and then crumple zones. Use the weakest magnet first and then work up to strongest. Once you get the feel a car can be checked out pretty quickly.
It's also possible to "map" a car using a infrared thermometer
Good shops will use only a skim after body work. I believe the manufacturers of the product say no more than 1/4".
I'd prefer to see 1/8" max at thickest.