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There are 2 ways of telling the first I do not recommend, start drilling your tank if it shatters it was tempered ;D Contact the manufacturer and give them as much info on the tank as possible, they will be able to tell you whether it is or isn't.
Many manufacturers place a tag on the inside of the bottom frame, so that you can see it if the tank is empty or bare bottom. Usually you see the common tank sizes like a 55 using tempered glass. If the glass looks thin compared to what you'd expect, it is often tempered.