Most window and plate glass manufactured today is done so with a process that floats the molten glass on a pool of tin (hence the name float glass). The surface of the glass that comes in contact with the tin will contain traces of tin and tin oxide.
There is usually sufficient tin contamination on the tin side that it can react with colored glasses or enamels that are fused to the surface. For this reason, glass artists who work with float glass usually want to know which surface is the tin side and which is the air side.
To identify the tin side of a piece of float glass shine a short wave ultraviolet (UV) light on it. The tin has a higher UV reflectance than the glass and will appear white and cloudy.