There are marshals (non playing officials) and there are PDGA marshals (member of the PDGA's marshal team). I don't think there is an official selection process for the PDGA marshal program. You are just asked by some other member of the team to help out (after showing that you'd be a good candidate). Thorough knowledge of the rules, the ability to diffuse situations, and being able to stay calm and assertive are important criteria. I believe that Mike P., Cris B., and Todd A. are all current PDGA marshals. If you want to get involved r.e. marshaling, your best bet would be to start with the state coordinator or talk to one of them.