55 gallon longs are 100% tempered glass, base, back and sides. This is due to the glass thickness used which requires extra strength from tempering to support the load.
You can check for tempered glass with polarized lenses as stated but you need two lenses. One on each side of the glass, then when you look though both lenses you'll see the stress structure in the glass created from the tempering process. For an example you can experiment on your car to see the difference. Windshields are not tempered but all other windows are.
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