I think pretty much all newer tanks over 10 or 20 gal
are tempered. If you have an older tank, you might chance drilling a hole, but even then, you might get yourself into trouble.
In general, non-tempered tanks have thicker glass. Eg. my new 80 gal
is 3/8" thick, but the old (20+ yr) one is over 1/2". However, I don't think thickness is a definitive guide, so unless you know for sure from the manufacturer, you are taking a chance.