I'm really enjoying learning about the older bikes but I must confess to feeling a bit lost!
"Its all in the lugs" Lets say you are looking at a naked frame with out paint or parts. Examine the head and seat lugs. A quality frame will have noticeably thinner mere elegantly formed lug work. Also the brazing will be neatly performed with little slop. Finally the lug and surrounding tube will be neatly filed so that he whole joint is a smooth elegant transition.
Now look at the bottom bracket shell, on a cheap bike the tubes will join to the shell in a blunt no frills joint, but on a quality frame the BB will be treated with the same artistry as the head and seat lug.
There are other details like special cut outs but that is another chapter
The next installment: filet brazing
I was old school before it was cool