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Hope this helps it does work well, tricky part may be finding some polarized sheets/film:
This is a direct quote from a good old post on the Aquatic Plants Digest:
Take two photographic polarizing filters, and Sandwich them together. Rotate one of the filters. Light transmitted through the filters will fade from lighter to completely blacked out as the planes of the polarized material crosses through each other's axis. Now, try the same thing with the piece of glass sandwiched between the two filters, and rotate one of the filters. If the glass is untempered, the light transmitted through the filters will act the same way, fading from lighter to blacked out. If the glass IS tempered, an interesting phenomena will occur: as the filters are rotated, a black cross will form through the filters. It's an unmistakable sign: if the cross is there, the glass is tempered.
fyi, I believe the phenomena described with the polarizing filters occurs due to the way in which the glass is cooled at the final stage of the tempering process. After the glass is heated it is air cooled by many jets of air that are moved either in a linear back and forth motion or a circular motion. The first results in noticeable linear lines in the glass when viewed at the right angle and the latter results in an evenly spaced disc pattern.