you would know what brand it is by knowing what it was when you bought it.
biggest 3 in US are All-glass, Oceanic, and Perfecto (all-glass and oceanic are the same company).
other than that someone might know by taking a look at the sticker if its of a popular make Im sure someone could identify it.
with a tank that old I would only be worried about how the silicone is keeping up and whether or not its had water in it the whole time. Generally silicone will dry out if its not used for an extended period of time and cause problems like leaks and sometimes (if bad enough) can "unzip" an entire corner of the tank.
EDIT: If you DO buy it I would seriously leak test it. What I do with any new/used/out of action for a while tank that I have is put them on a level surface outside (I use my garage) and fill them up with water and let them sit for a while and investigate for leaks. Worst case scenario is it leaks/breaks and water goes outside...better than finding out in your house.
