How to tell if glass is tempered?

So I could pick up a 120g with stand for 130 bucks. It's not drilled so I was thinking either a hob or drilling it and getting a glass holes overflow. How do I make sure its not tempered glass? Are most back panes not tempered?
From what I remember the bottom pane is what is tempered. I need to make sure though.
The seller said he was unsure.

 

There are a couple of techniques by which it can be determined whether your fish tank is tempered glass. One of the way to determine the character of the glass is through use of polarized sunglasses, which enables seeing a fuzzy blackish diamond or circular pattern in the tempered glass as in a car window. LCD screen and a polarized filter can be used to determine if a sheet of glass is tempered. LCD screens also have a polarized filter set at a 45 degree angle across the screen. The stress in the tempered glass causes light to polarize in a pattern within the glass, which is seen through the polarized sunglasses. You can also place LCD screen behind the glass and look at the screen through a polarizing filter. If the glass is tempered then the LCD screen would be seen as though having lines and stripes. In case it is a plain un-tempered glass, the LCD screen would be absolutely clear.

 

Weird way but its how i checked mine... If you don't have 3d glasses then its a good reason to go pictures lol

Tempered Glass Test - YouTube