[Aegon the Unworthy]

Joff.

Tyrion tricked him into revealing it with a flinch (Joff not Tyrion) after Joff uttered "I am no stranger to Valyrian steel" during the gathering before his wedding. You know after Joff chopped Tyrions gift in half and told him to get another present for him.

Its believed but not confirmed really that Robert uttered something like "It would be a better thing to put that boy out of his misery". So Joff, always seeking approval with his father took the knife from his fathers baggage train, hired a camp follower and had the deed done. Maybe the Hound acted as an intermediary or would know the truth....other than that we'll never really know.