You won't know if it is a junk tank until you get a visual inspection and a hydro, but you can ask some questions about the valve.
What kind of valve does the tank have? If you dive DIN and it's a Yoke, that would be a problem.
Next, will you need to get it O2 cleaned for partial pressure blending of Nitrox? Well, if so, you have another valve concern. Many older tanks have a different size opening for the valve. This is important because when you get a tank O2 cleaned, the valve gets rebuilt. Many of the valves on Genesis tanks no longer have valve rebuild kits. If you can't get a rebuild kit for your older valve, you may have to buy a new valve. If your new bottle has an exotic valve, you may have a failed project on your hands. So... research the brand and make sure the bottle and valve can be made to work with your needs.
Might as well make sure it doesn't have a funky J-Valve, unless you like that retro adventure diving.
If you are a DIR kinda diver, you will already have many preferences for valve types (no hard plastic or metal knobs. Short, hard rubber knobs preferred)
If you get a look at the cylinder, give it a good look. If it is galvanized, great! No scratches are permitted in the galvanized coating, no rust.
Look for rust all over, especially at the neck where the valve joins the bottle. Make sure the bottle and valve are not loose. Make sure that there are not any stickers or bands that impede your ability to inspect for rust. If the tank is painted, make sure that none of the paint is bubbled up. Be wary of anything that can hide rust- look under everyting.
Be happy if the tank has air pressure in it, but never discount that the seller may have put a little air in the bottle just before you arrived.
You cannot count on using used bottles to build a doubles set- so assume that this bottle will be used as a single.
That's about it. Good luck.
