My own definition - proto-science is something put forward by someone who knows the scientific orthodoxy in the field, suggesting that some idea might be true. Pseudo-science is something put forward by someone who doesn't know the scientific orthodoxy, asserting that something is true.

This seems like an excellent heuristic to me (and probably one of the key heuristics people actually use for making the distinction), not not valid as an actual definition. For example, Sir Roger Penrose's quantum consciousness is something I would classify as pseudoscience without a second thought, despite the fact that Penrose as a physicist should know and understand the orthodoxy of physics perfectly well.