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?

I would just look for obvious stuff like cracks or rippled or bent tubing. Even if it has a little rust its still ok. Even if its bent a little you can bend it back. What kind of frame is it?

Um . . . Harrier I believe is what the badge said. I don't recall if it had a model name or anything. I remember the paint seemed pretty much in tact, the tubing looked strong (no ripples) and the lugs looked good considering the presumed age of the bike.

Beware, not all lugged bikes are of any quality. If its a heavy bike, its possible that its made out of really cheap tubing, lugged or not, and isn't worth its weight in lead. Also look at the dropouts. Are they a good 1/2" thick or so like many road bikes have? Or are they 1/8" steel plate? That will tell you alot about the quality of the bike. If its a good frame, and its straight, have fun!

Good dropouts are also moulded rather than stamped so look for some curvy detailing. Even if the tube labels have worn off, dropouts are a sure sign of bike quality.
Bike frames tend to fail in non-critical parts such as the drive-side chainstay. It is rare for a steel bike to fail catastrophically and dump the rider. You just notice a noodly feeling and behold, your chainstay is waving in the wind.