If you're able to determine the tank manufacturer, that's a good starting point for knowing if the glass is tempered and which parts, bottom, side, etc. There is another method using polarized sunglasses and an LCD screen with white screen behind the glass you're trying to check. There should be a distinct change in how the screen looks when you rotate it or the sunglasses if the glass is tempered, generally a pattern or lines will become evident. If it is not tempered, there will be no change in how the screen looks.
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