Shawn Spencer actually does have psychic powers, and always has, but back when he was a kid Henry decided to push him into becoming a cop, with training as in canon.  Could go either way, either Shawn's aware of it all along, or Henry keeps it a secret from him and convinces him that it's just photographic memory and extensive training.  Because I've seen a couple of "Shawn gains psychic powers" fics, but I don't think I've ever read one where he's always had powers.