Caveat Emptor, dude: you really have to look at it like Sherlock Holmes when buying or selling; unfortunately most of the time, a 'vibe' is all you have to go on- unless of course you either fully remember or carry the National Stolen Bikes Registry with you.

For the ethical, the best solution to set your minds at ease is to check any newly purchased frames against the Registry and- if you're truly ethical- you'll return anything found to be stolen to it's original owner, despite how much you've paid for it.