Tempered glass, also known as toughened glass and safety glass, is glass that has up to five times the strength as normal glass. Additionally, the glass is designed to break in small oval pieces that are significantly less dangerous than the jagged pieces regular glass breaks into. Tempered glass is used where human contact with glass is high (i.e., doorways and car windows). It's difficult to tell if a piece of glass is tempered, but there are a few indications you can look for.
Put a pair of polarised sunglasses on and hold the glass up to a light. The polarised lenses of your sunglasses will reveal a pattern of hardening on the surface of the glass. This pattern is distinctive to tempered glass. You'll clearly be able to see that the pattern does not go all the way through to the core of the glass.
Feel the surface of the glass for a slightly rough edge. Tempered glass is treated with heat to get its strength. Sometimes this process is done incorrectly and gives the glass a textured, slightly rough feel.
Look for the smallest cracks or chips in the glass. Tempered glass is made to completely break apart if even the smallest crack or chip occurs.
Look for a slight bend in the glass. The heat treatment tempered glass goes through sometimes creates what is called a small bow. You can look at the glass from the sides to see if there is a slight curvature to the glass.
Some tempered glass come with an etching in the corner indicating that it is tempered. This would be extremely helpful except for that fact that the etching is usually covered if the glass is set in place.