Re: A couple that are easy to spot:
If we're talking about John Wayne specifically he didn't play a real cowboy all that often (maybe in the Republic era?). There are still lots of westerns that are about actual cowboys or at least take place on or around a ranch where you get get to see lots of people who are supposed to be working cowboys.
Most cowboys would also be pretty young. And they didn't call them "cowboys" back then. If I remember right they would be "drovers" or "cow hands." And calling somebody a "cow poke" would be fighting words, since "having a poke" was a euphemism for sex.