Bingo! Not the player, the DM. If the player says he remembers what his Troll slayer uncle told him, the DM decides whether that happens without a roll or requires a roll, not the player.
As long as the player indicates an action, such as recall, he doesn't have to phrase it as an action. If the player says, "I remember my uncle telling me about trolls.", that's an attempt to recall, even if not phrased that way. Recall of lore is an ability check if the DM says so. The DM does have something to adjudicate at that point.