Contact the manufacturer of your aquarium and ask. Generally any standard tank under 30 gallons won't be tempered at all. Any larger and some brands and sizes begin to have tempered bottoms. All-glass and Hagen tanks also have warning notices stuck to the bottoms.
Also, like Rusty said, tempered glass generally has nicer looking edges. Plate glass is often just ground a bit to make it unlikely to cut you, but it looks rough (look at the edges of a small rectangular tank vs, say, your coffee table).
Beyond that, if you want a real way to tell you'll need a microscope or some kind of expensive and complicated gadgetry. In short: there is no way to tell. Or, rather, no way I've been able to research. Well, I did find this...
http://www.astronomy.net/forums/atm/messages/3492.shtml