It can be quite difficult from a superficial view to know if the glass is tempered, but few tricks will help you identify it. 1. They have smooth edges. This is done because once the glass is tempered, it's nearly impossible to work with it. 2. Watermarks are present on tempered glass for safety approvals and contain information about the manufacturer. This is a good indicator for tampering. 3. Look for heat stress signs like dimples and imperfections. They are a good clue to know about tampering. 4. If the glass is tempered, putting on polarized glasses and seeing the glass under the sun creates the illusion of parallel lines.