Some Tempered Glass has a "sheen" to it. i.e. It has a slightly different color when looked at from the side. Most of the tempered glass sheens I have seen has a greenish cast to it.
Also, does your tank have a tag on the bottom stating whether any panels are tempered?
How big is the tank that you are thinking about drilling?
A single pane of tempered glass is also heavier than a non-tempered glass pane of the same size.
I know this won't help you in this exact case, but just what I have observed.
EDIT: Oh, I totally agree with Paul. Once you have shattered your first piece of tempered glass, you will never do it again.
One pane of 36x36 tempered glass later..... :mad2: I learned my lesson. I now mark my clear glass, when I first get it as to whether it is tempered or not.