If it's stamped, the last digits may be stamped above the frame right wheel arch. This started in the late 70s so there may be a number there.
See here. (Link)-> How to ID a frame -<
I want to do a scratch build and found this frame for sale. They think its an 80's era frame but not sure of the model. How would I figure this out? Also, what is a reasonable price on a frame?