How would I know if it's tempered by using a lens?
You hold up a polarized lens to view the tank and hold a laptop or phone screen behind it. Then you rotate the lense and if the the screen turns completely black then it's not tempered. If you see a lot of lines then the glass is tempered.
This is a picture I took when I used a polarized camera lense to check my tank. Notice how looking through the lense the screen is completely black with no lines of light shining through? That means the glass is not tempered.