Just as a small hint, it may also be worth the time to "train" your eyes to see the difference. I suggest taking a look at a normal piece of glass in a window at some point away from entryways, assuming it is not tempered and then check near a door as those are often required by code to be tempered. Once you can see the difference, then is a good time to check the tank as it may be missed when not knowing exactly what to look for on the difference. Perhaps just a check of the bottom versus sides will give you the right "experience", also? To verify, the tempered glass in houses and cars will often have a small label on them.