When I restored my broken $50 CL 244 tank with a new 24"X72"X1/2" bottom panel panel of special ordered tempered glass, the edges were beveled and the corner embossed with a logo signifying that it was tempered.
Another way to determine if it is tempered is to swipe it with a razor blade as most tempered glass has micro pits developed from the flash cooling of the outer surface at the factory. When the razor blade strikes a pit, tiny micro shards will form scratches. Why you should never use a razor or strait edge to clean tempered glass as the super hardened outer layer is extremely hard and brittle and much more easily scratched with hard objects, and why you should only use it on the bottom, never as a side viewing panel.
As far as using a hammer the edges of a tempered glass panel are the weak areas and while you can strike a tempered panel several times full force with a large hammer on the flat sides (compared to once with non-tempered) without breaking, the edges are very vulnerable and will shatter with only a few taps of a hammer on the ends.
Why I used it was after seeing how vulnerable bottom non-tempered glass is on a large panel, I opted to spend $30 more for tempered and it has paid off not using egg-crate have dropped a few big heavy stones with no ill affect and also becasue the bottom carries the most wait of the other 4 panels, sand, rocks and main weight. The extra $30 was well worth the 3X to 6X increased strength.
http://www.youtube.com/watch?v=aupoGfnvScs