I think he will either see Merlin using magic or he will figure it ourt on his own accord. I also think it will take him a while to tell Merlin that he knows because he is 'lost' and doesn't know what to do. I'm possitive that he won't tell anyone, not the king, not even Gaius!

I'm pretty sure he'll just tell him like in the legand or Arthur will see him which ever happens first

I think that....
Merlin will use his magic to save Arthur in some life-threatning situation, and then Arthur will ask if it was a new thing or has he always been able to do that...ARthur will keep it a secret and when Uther die's, Magic is welcome..ta da!!

Awesome Shepard14 i love that idea.

I think he'll find out because Merlin shows him.. I don't think Arthur would believe Merlin if he just told him.

Think about it... to save Gwen in season one episode three, Merlin tells the entire court, uther, and arthur that he was the sorcerer who caused the plague and not Gwen. Everyone just denounced him to being in love, so they obviously wouldn't believe him if he just said it! I think, one of these days, during one of those times when they're fighting some beast and arthur happens to be unconscious while Merlin slays it, maybe arthur will wake up EARLY this time and actually see Merlin do it! that woud help alot! oh idk, I trust the writers. whatever they decide is for the best. except that they haven't decided Merlin and Morgana need to be a couple, and that would be for the best. Otherwise, i trust them! lol