Quoted from SirScott:I'm not sure if this really works, but might be worth experimenting.
http://glass-fish-tanks.com/how-to-tell-if-your-fish-tank-is-tempered-glass/
From that site...didn't realize tempered glass has such powers as to prevent my hands from moving! No wonder pinball is hard it's the glass I tell you! 
"If you put your hand in front of the fish tank which is made of tempered glass, then whilst viewing through the polarized glasses then you should not be able to use your hands due to reduction of reflected light."
Fairfax, VA
