Hold them up to the light, look at them and say "ooh", "aah", or both (at the same time if you're really good)![]()
My method is to chop them up into printfile holders and then hold them up to the glass of my window, and look at them through a loupe, because I'm too stingy to buy a light box.
The ones that then look good I scan and have a better look at, no need for glass plates unless you want to keep them mounted and think you might damage them in a normal mount