simplest (worst) case: Treat it like any tank that is full of something that you have no clue what it is. it could contain almost anything. but not likely, do you feel lucky?
I would take it to your LDS and explain your situation. they will likely err on the side of extreme safety and bleed it out, remove the valve and perform a visual check to ensure there are no hamsters inside. They will then fill it with an appropriate gas mixture for you.
this is a simple, relatively cheap, well known procedure that will give you complete piece of mind as to the current state of the mystery stuff inside the cylinder.
on the other hand: if the tank comes from a reputable source that you trust, just dive it.