Look at it through crossed polarizers - tempered glass usually shows a lot of strain bifringence. Get some polaroid material (cheap plastic clip-on shades will work) and view through the glass with one sheet on each side rotated 90 degrees to each other. For instance, clip on one pair to your glasses, hold the other polarizer in front of your light source and rotate it until you get maximum extinction. Now insert the glass in question between the polaroids. Tempered glass will exhibit distinct patterns of dark and light bands or other shapes. You might even get groovy colors. If you can see through the glass edge-on, then it will show a dramatic pattern of colored bands parallel to the faces. If you see no patterns, and light is evenly extinguished as you look through it, then it is not tempered and is pretty well annealed.
Finally, you can examine the glass closely for any brand name etched into it. Examine the edge for any sign of fire polishing. Examine the surface for little dimples where tongs may have indented the glass. Any of these indicate tempering, and you should not try to cut or grind this glass, or you will have a terrible mess.
