How to tell if pre-war? Lionel.

If you have a Santa Fe diesel, there's a pretty darn good chance that it's postwar.

Aside from that, there's one big difference between prewar and postwar trains that's easy to spot. All the postwar trains I know of have knuckle couplers, which are the ones that look like couplers you'd find on real trains. All the prewar trains I know of have some sort of hook coupler, with a hook perpendicular to the track and some sort of box it fits in to. 

Also, a good majority of prewar cars were made of tin plated steel and painted in bright enamels, while most postwar cars were cast plastic. There were a few plastic prewar cars, though, and some of the tinplate cars(namely a few passenger cars) carried over to postwar, but with knuckle couplers.