Tempered glass is treated with a special heating and cooling process to increase its resistance to stress. This treatment also changes the way the glass breaks, making it better for use where safety is a concern, such as in doors and car windows. The International Residential Code specifies that all safety glass be marked by etching, embossing or ceramic firing so that the label cannot be removed or destroyed. Viewing the glass through polarised glasses can also identify it as tempered.
Inspect each corner of the glass to see whether it has an etched logo. This may be covered by a frame. The logo may contain wording indicating that it is tempered glass, along with the manufacturer's identifying information.
Look along the surface of the glass with light reflecting off it. Tempered glass may have a very slight wavy surface, caused by the rollers used to move it through a horizontal surface. If this is not apparent, the glass may have marks on one edge where tongs held it when moving through the furnace.
Put on the polarised glasses and look at the glass at an angle. A vague checkered pattern is visible at the right angle on tempered glass, which is a result of the tempering process.