I'm guessing it might have used to be a shower door?
before you even think about cutting it, try to figure out if it's tempered. Easiest (but least likely to provide any info) would be to look over it for any etchings/markings stating it's tempered.
Failing that, I'd look along the edges for that characteristic conchoidal fracturing. Finding some wouldn't necessarily mean it's not tempered, but it would be some evidence towards that.
There is a way you can check for whether glass is tempered or not using polarized lenses (you can get these from the 3D glasses in movies). I'm not sure on the specifics for checking tempered, but play with the lenses alone first, then with a piece of non-tempered glass, and then your new piece. Stack the lenses on top of each other, and look through both of them, and rotate just one of the lenses while you do this. There should be a point where they are almost completely black, and then it should get brightest about ~90 degrees from that arrangement.