My girlfriend's character, Elven, has Sixth Sense and so I've been thinking a bit about how to GM this (and similar skills in other game systems). While GM'ing Elven the other day I had this idea.
The idea starts with the theory that a player is always in one of three states:
(1) “Normal” day-to-day life and roleplaying. The character doesn’t perceive any danger, the player doesn’t perceive any danger, and the GM isn’t about to pounce with someone who’s sneaking up on the character or laying in ambush. This includes conversations with strangers if there is no reason to be concerned. In these situations, obviously, Sixth Sense is not kicking in.
(2) “Unforseen danger.” You’re traveling down the road and there are goblins with crossbows hidden in the ditch. The patron who enters your shop intends to rob you. A shadow assassin is tracking you through the woods. You're skipping along the trail and there is a venomous snake laying in wait. This is what Sixth Sense is designed to help you with. Is it fair to say that the typical result is that you're "not surprised" and therefore you're saved from making a Steel roll?
(3) “Suspicious" or "Obvious danger”. You see an orc (and you're not an orc --- or you are an orc and you see a dwarf!). You reach the treasure room and, well, all adventurers know that treasure rooms are trapped! You’re opening a coffin in what you think is a vampire’s hangout. The guy across the ghetto alley is staring at you intently and flicking his dagger. Sixth Sense won’t help you here because sixth sense will always go off in these situations whether the situation is dangerous or not. Sixth sense is not a defense against paranoia. If you think the situation might be dangerous, your sixth sense goes off, whether it’s warranted or not. You can’t use this trait to figure out beforehand whether the vampire is in the coffin.
Basically, the trait is useful primarily as a thwart to case 2, and I believe that's sufficient to justify the cost. But if you already suspect there might be danger, your sixth sense won't help you (as a player) know if your suspicious are warranted.