Found it: Page 296 of the Atlantean Edition.
Invisible creatures require a DC30 check to notice. Inanimate or unliving invisible things/creatures require a DC40, but it is a +20DC bonus to use Spot ro pinpoint the location of something invisible, and even on a success, the check would succumb to the rules for Total Concealment (50% miss chance as if attacked).
Listen is the only way to do it based off the invisible creature's Move Silently check. You can make a Listen check every round as a Free Action. Groping around for it works too, but obviously has some drawbacks if it's a monster (!).
It continues onto p297 with more specifics on locating invisible creatures and objects.
I believe that since the only way to fundamentally detect an invisible monster, then, is to Listen for it as an action, one must assume that at all other times it is completely impossible to tell where such a creature is. During a combat, I'd assert that Listening is harder, so I'd penalize significantly unless the character used a Standard or Move action to Listen instead of the normally allowed Free Action (that's me houseruling here).
But, the gist is this:
- Concealment is confered by Invisibility.
- Concealment allows for Hide Checks to be made where otherwise Cover would be needed.
- Successful Hide Checks aford the ability to Sneak Attck from that hidden position.
- Therefore, Invisible Creatures can sneak attack if they have that ability.
The big thing is that you cannot Spot an Invisible creature, only hear, smeel or touch it. Youalso cannot sneak attack an invisible creature because of the Concealment rules. Concealment states that you dont'actually attack the target, but instead you attack into it's occupied square (thus the 50% miss chance).
:wink:
But, this isn't "Genereic Spotting" anymore, so I suggest we either get back on topic or start another thread to discuss Spot in detail.