There are a few rules-of-thumb for what is a decent frame and what is crap.

the following things are found a lot on low-end ten speeds from the bike boom era: stem shifters, suicide brake levers, cottered cranks or one piece cranks.

Indicaters of nicer frames could be tubing by Reynolds, Columbus, Vitus, Tange, etc.; forged drops as opposed to stamped. Also look at where seat stays meat the seat tube lug. On nice bikes this connection point is done quite well. I don't know lug terminology that well, but this is usually an obvious charecteristic.

When in doubt just ask over in C&V. They are an incredible resourse for this stuff.