BTW a player pointed out to me that, somewhat ironically given this discussion, on PHB pg 81 "Sense Motive" it is clearly described as an active skill which the player has to choose to use. The GM may make the roll in secret so you don't know if you succeeded, but from the wording it's clear the player has to request the roll. Furthermore "Action: Trying to gain information with Sense Motive generally takes at least 1 minute, and you could spend a whole evening trying to get a sense of the people around you."
So according to the book RAW it can't even be used in "He went thataway!" situations, even by player request, unless you're prepared to spend a minute or so interrogating the speaker. You could make it that detecting Bluffs is not an Action, which seems reasonable to me, but there's nothing in the description to specifically support that ruling.
So according to the book RAW it can't even be used in "He went thataway!" situations, even by player request, unless you're prepared to spend a minute or so interrogating the speaker. You could make it that detecting Bluffs is not an Action, which seems reasonable to me, but there's nothing in the description to specifically support that ruling.
