Identifying tempered glass is easy if you don't mind breaking it. Unlike annealed plate glass, which shatters into large, dangerous shards, tempered glass fractures into bite-sized pieces with much less ability to inflict personal injury. You probably prefer to keep your glass intact; even so, you can recognize tempered glass by its distinguishing features.
Look for the Mark
Plate glass is usually annealed by heating it and cooling it slowly to make it less breakable. Manufacturers make tempered glass by reheating annealed glass to a temperature of about 1,200 degrees Fahrenheit, then quickly quenching the heat with air currents to bring the glass to a temperature of around 400 to 600 degrees Fahrenheit. Because tempered glass has characteristics that are so different from annealed glass, manufacturers are required to identify it with a stamp in at least one of the corners. The stamp, which may say "Tempered" or simply "Temp," is either sandblasted into the glass or applied as a porcelain overlay.
Examine the Edges
Because tempering requires extra processing, the edges of tempered glass are usually completely smooth, while those of annealed glass are often ridged, scuffed and imperfect. Run your finger along the edges -- if they are exposed. Any roughness you feel means the glass probably isn't tempered, provided the glass is in good condition and the roughness isn't the result of abrasion incurred as the glass was used.
Look for Surface Scratches
When a pane of glass is mounted in a window, the edges usually aren't available for examination, and the manufacturer's mark my be hidden by the frame, or it may have worn off. In such cases, look for small scratches on one side of the pane. If any are visible, they were produced by debris left over from the tempering process. If the glass wasn't properly cleaned before heating, small particles melt and fuse to the surface on the side of the glass that's on rollers, and this debris gets dragged along the surface of the glass during normal cleaning to produce the scratches.
Get Some Polarized Sunglasses
When you view tempered glass outside in the sunlight using polarized sunglasses, you should be able to see dark, shady lines stretching across it. These lines were formed by the rollers the glass was resting on during the tempering process.
Score a Line
In the absence of other identifying features, you can tell whether glass is tempered by scoring a line on it with a window-cutting tool -- but do this only if you plan to cut the glass anyway. If the glass is merely annealed, the tool makes a clean, white line, but if the glass is tempered, it makes a glittery, uneven one, and material will sliver and flake off from the score line. If this happens, don't try to cut the glass yourself -- it will chip and crack. Take it to a glass specialist, who may be able to cut it after annealing it once again to remove the temper.