Does anyone know if there is a method to test window glass that has already been installed to determine if it is tempered?
Yes, there are both quantative and subjective tests available.
1. The surface compression of the tin side of tempered float glass can be measured using a grazing angle surface polarimeter (GASP). This is most easily done on horizontal flat glass, but is often successfully accomplished on vertical glass surfaces.
2. Fully tempered glass will exhibit optical fringes (dark shadow lines) when viewed between two polarizing filters. From the appearance of those fringes, an expert can often estimate the approximate degree of temper and even identify the furnace type used in the tempering process. It is not possible to measure residual stresses in this manner.
3. It is possible for someone with the requisite expertise to assemble a full polariscope by placing two polarizing filters, and other filters on both sides of an installed glass plate. Then, the net stress through the thickness of the plate is measurable. (The filters must be ruled and calibrated so the rotation of various filters can be accurately measured.)
All of the above-mentioned tests are non-destructive. They are generally applicable only to monolithic glass (not insulated or laminated glass where multiple plates of glass and/or other birefringent materials are present.
I conduct such tests routinely in our lab. If you need further assistance, let me know.
If the tin side of the glass can be identified and accessed, the Strainoptics GASP(R) surface polarimeter can be used to measure the surface stress of the tempered lite. This will provide a quantitative measurement of acceptable surface compression for tempered glass per ASTM specification C1048 and test method C1279. For a qualitative inspection of the typical tempered glass stress pattern using polarized light, the Strainoptics PSV-100 portable strain viewer used in reflective mode will provide a reliable indication.