I'd suggest looking for a sticker on the downtube right next to the head tube. That sticker will tell you the frame material. Reynolds 501 or 531 is what you should hope for; that's the good stuff. I don't know how valuable HLE or Carbolite frames are.

I'd definitely suggest reading that page on the sheldonbrown.com site - if you know a bit about the bike, you will have more authority with which to haggle. Good luck!