My first big tank (180 gallons)

I've never refurbished an aquarium before so I cant help you with that. You can tell if something is tempered using polarized glasses. Take it out in the sun and look for darker/shady lines running through the glass. If it has these lines then it is tempered. Another way is to shine a lcd screen (we used an portable dvd player. If you see rainbows in the glass then it is tempered. I would make sure that you have identified tempered glass somewhere else (often times you can find tempered glass near doorways or in your car) so that you know what it looks like and dont get a false negative because you don't know what you are looking for.  I will add that it is very common for tanks to have tempered bottoms. I will also add that my trust in used tanks has deteriorated to the point that I don't use them especially in a big new setup because I wont sleep well at night. $1000 for a new tank is cheaper than replacing the floors. Then again, no tank is immune to the possibility of failure.