Most likely yes... But put a laptop computer with an LCD screen in the tank (with the tank empty of course) and rotate the screen. If the screen gets funny lines in it as it is being rotated and you are viewing it from outside of the tank the glass is tempered and can NOT be drilled.
Tempered glass polarizes light. Viewing a polarized LCD screen through the polarizing glass blocks some of the light waves when the two polarizing effects are at 90 degrees to each other. You see the blocked light waves as funny lines on the screen.