Tempered glass should have a little white/greyish label near one of the corners stating that it is tempered. You can find this label on storm doors and on your car windows. Most glass fish tank are not made of tempered glass. I broken a few 10 gals and they don't shattered like tempered is supposed to. I do not know about larger tanks though, but my guess is no, they are most likely not tempered. Also, cutting glass is not an easy task. I made many mistakes, costing me a lot of $$$$ before I got the hang of cutting glasses right.
It may or may not indicate that it is tempered. Often the bottom pain of tanks is tempered. Sometimes the other panes are tempered. There is no simple way for the average hobbyist to be able to identify if it is or not.
By looking at the glass while wearing polorized sunglasses you can tell if it is tempered as you will see a fuzzy blackish diamond pattern in the glass. You might recognize this from looking out your cars rear window while wearing polorized sunglasses. While not a sure fire method, if you do see the pattern, it is tempered.
Glass up to 1/4 inch is quite easy to cut, just remember to dip the cutter in oil, don't score too deep and snap immediately. If you have a practice piece, even better! Over 1/4 inch, I'd have cut for you unless your willing to risk it.