Originally Posted by Retro Grouch
Look at the bottom of the seat tube. If the manufacturer was proud enough to put a sticker on it to advertise the material that the frame was made from the chances are it's a worthy bike to work with.
Good advice.
Given that we're probably talking about bikes more than 15 years old, your best bet is to stick with steel. Older aluminum and carbon frames are MUCH more suspect. Look for stickers indicating frames made from double-butted, cromoly-steel tubes manufactured by Reynolds, Columbus, or Tange (in no particular order). If you find such a sticker, you can be 99% confident that the machine was at least of reasonably high quality. You might even find some real beauties. Assuming no rust, you can set yourself up with a very fine-riding machine. Just keep in mind that such projects rarely make economic "sense" -- but what labor of love does?
--Doug