When looking for frames - especially with older or very neglected frames, there might not be a nice sticker telling you what the frame is made of. Obviously, one would first ensure the frame is sound, but presuming that it is, look at the right rear dropout. If it is stamped - it certainly is not a high quality frame. If it does not have a separate hole for a derailleur to attach to, it almost certainly is not a high quality frame. If it has an adjustment screw (or the hole for an adjustment screw), it is almost certainly at least a mid-level frame (ie - not junk).
If you know what kind of bike it is, and have time to do research, you can look it up on the net (or ask T-Mar)
