You might also want to take it to your local glass shop and let them check it. If it is not tempered you could have them get you a piece. My last trip to local glass shop revealed that the glass was plate glass and a new piece of tempered glass was only $33.00 (plus tax).
Also, look at the edges and if there are any chips along the edge then you can be pretty certain it is NOT tempered. This is because tempered glass can withstand a lot of force to the glass however, even a slight force to the edge will cause it to shatter.