juliaromero said:
Does the scent ability work automatically, or do you have to make a spot check to notice something (with some sort of circumstance modifiers?)?
Within the normal range (15' to 60', depending on wind and modifiable by creature), it works automatically. That is, if something smellable approaches within that radius, the creature with Scent knows it. If the creature comes within 5', the Scented creature can pinpoint the sqaure. (Note that this doesn't negate Sneak Attack or the like ... if a creature can't see an attacker, the creature is still denied its Dexterity bonus to AC.)
It does take a standard action to determine the direction of the scent, but there's no skill necessary.
I've seen house rules where people invented a skill -- automatically a class skill for creatures or characters with Scent -- and in certain cirumstances that works fairly well. For instance, when detecting scents far outside the normal radius, such as a distant forest fire, how does Scent work? Automatic? Maybe. But I could also see it being a perception skill similar to Spot or Listen.
In my game, I just use the normal rules, and assume that distant strong odors are no more difficult to detect for a creature with Scent than a mountain 30 miles away is difficult to detect for a creature with eyes, even in the absence of Spot skill.