wiggy:
Not sure about the US but dare I ask what exactly are people expecting a doc to present when asked for their "credentials"? A business card with "Doctor" on it proves nothing.( and as mentioned in the other thread in some circumstances a para medic may be more use than a doc)....
A wallet size license card issue by the state medical board.
As to paramedics, my son-in-law is one. There isn't a whole lot he can do except by instructions from the ER physician via radio link.