"If you tap the playfield glass and do not hear a 'thump', but rather a 'tinkle and a rattle', you need to replace the glass."
Old owners know what I am talking about here.
You use your ears.
You can also easily determine tempered glass via use of polarized optics from the the process that @bbreise was referring.
You use your eyes.
"Another way to identify tempered glass is to view the glass through a pair of polarized sunglasses. In many cases, viewing the glass in sunlight, through polarized lenses, will reveal dark lines or spots. These are prime indicators that the glass is tempered. The lines are formed by the machine rollers during the cooling process."