ORIGINAL: Lifer
Trial and error. Eventually you will have a drawer full of props and you can try them on all your engines/airplanes.
In one case at least, it is a car trunk that's not full (or the airplanes couldn't get to the field) but has a plentiful pile to one side. And when I can convince them, the guys at the field will have a good selection to try on their own models. (And of course, I get to try them on mine.)