Well for American bikes it's very simple, it's those stamped dropouts and those lugless frames - the schwinn's with their forged forks and smoothed frame joints - columbias with their weird U chainstays, American bikes are very distinctive.
The old Japanese bikes generally seem to be small, and the really old ones remind of british bikes because the nipponese, like the brits loved wacky kickstands and rod actuated brakes. The Japanese bikes of the 50's and 60's I think are probably some of the coolest out there.