I was worried about my 65 gallon tank when i drilled it.. The bottom said tempered glass, but that was all.. Non tempered glass will have a greenish look to it, if you just look at the sides of it.. Tempered glass looks gray..obviously you cant look at the sides of the bottom of the tank, but you can see all the rest of the edges of the tank.. I also just used a polarized pair of sunglasses and a laptop to double check the glass i was going to drill.
Here is a link on how to check the glass
How to tell if glass is tempered.
Now that i have done this once, i will never have a problem in figuring out if its tempered or not.. i can just tell by looking at the color of the glass now but this laptop/glasses test just confirmed it for me.. Good Luck