I was at a salvation army recently and spotted an old 10-spd steel lugged bike. I've always wanted to build a bike up from its frame, and this looked like this could serve that purpose. I was going to buy it, when the thought occured to me: how do I know if this frame will hold up? I mean, I could more or less tell that most, if not all, of the components were trash, but what about the frame? I can always search it over for signs of rust, but sometimes it's not that obvious. Are there other things I should be looking at when pondering the safety of a frame?