Easiest way to tell, take an LCD screen and shine a white background through the glass (use google.com) then look at it with polarized glasses. If it's tempered you will see color streams through out it at an ange. If I'm not mistaken all four sides are okay to drill but the bottom glass is tempered and will shatter if drilled. I personally have drilled all aqueon and Marineland size tanks from 20-75 gallons( about 80 tanks) on the sides for my store and not a single one broke. Not 100% sure on top fin tanks.