There is a way to tell if it is tempered, but I don't remember all the details. It has something to do with inspecting the tank outdoors in the sunlight while wearing polarized sun glasses. Apparently, tempered glass has a rainbow look to it.* Don't hold me to that though.
I would think stickers and such on new tanks would specify if tempered or not, but I only own second hand tanks so I cannot verify that. Best bet is to figure out what BRAND tanks are a part of the $1/gallon sale, then look at the manufacturers website and determine if tempered or plate.