I've been doing bodywork for fifteen years and the single most useful skill I've developed is the ability to read a panel with my hand rather than my eyes. Under shop lighting, minor waves and low spots are almost invisible. Under your palm, they're obvious.

The technique: lay your palm flat on the panel and slide it slowly. Your hand will rock over a high spot and dip into a low spot in a way that's immediately apparent. Work in multiple directions — a wave that's invisible when you're moving with it is obvious when you're moving across it.

Couple this with a raking light — a single bright light source held parallel to the panel surface — and you can see everything. The light rakes across every imperfection and casts a shadow that reveals the shape of the surface.