During last session the bad guys had Greater Invisibility and the party was also invisible but they knew of there presence and there was 2 ways in, so 2 of the bad guys cast fireball's on each entrance.
They said they should have become visible once they attacked and I said no, they have greater invisibility, and they were fine with that.
But one of them argued they should have been able to see whereabouts they were because they cast the spell, and in the spell description it says;
You point your finger and determine the range (distance and
height) at which the fireball is to burst. A glowing, pea-sized bead
streaks from the pointing digit and, unless it impacts upon a
material body or solid barrier prior to attaining the prescribed
range, blossoms into the fireball at that point.
So he says they should get a perception to see to point of origin of this pea-sized bead.
What kind of DC would this have been? It was after the fact of casting.
Not like they were actively perceiving.
You get a DC 20 Perception check to notice an invisible creature is in the area. A DC 40 Perception check to pinpoint which square. If the invisible creature is in combat, those both drop by 20. So to pinpoint the location of an invisible caster casting a spell is a DC 20 Perception check (modified by range and other circumstances as normal).
It doesn't matter if they are actively perceiving or not - most Perception checks are reactive, and require no action on the part of the observer. There is no facing in Pathfinder - if something happens in the area, everyone gets a free check to notice it.
A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Perception check. The observer gains a hunch that “something's there” but can't see it or target it accurately with an attack. It's practically impossible (+20 DC) to pinpoint an invisible creature's location with a Perception check. Even once a character has pinpointed the square that contains an invisible creature, the creature still benefits from total concealment (50% miss chance). There are a number of modifiers that can be applied to this DC if the invisible creature is moving or engaged in a noisy activity.
I would apply sniping rules for this.
I would argue they are stationary (+40 from invis) (-20 with the sniping.) then roll each of their stealth checks.
I would advice not letting the PCs know they are with invisible creatures. I would have them roll d20s knowing what their perception is against the the snipe check. Then I do spellcraft checks to determine the spell. If they failed everything I would describe fire explodes around them. If they made the spellcraft and failed the perp, a fireball spell gets landed on you. If they ask more about where it came from I shrug(this would most likely prompt them to cast detect magic taking three turns with more spells flying at them). If they succeeded the perp, I will say what square the fireball came from.
I found the best DMing rewards people for the skills they have or get lucky with. If PCs get unlucky or do not have the skills for the situation they know less of what is going on.
What kind of DC would this have been? It was after the fact of casting.
Not like they were actively perceiving.
I'd make it quite easy since it is glowing. It is fast, though, so I figure a DC 10 is about right. That would reveal where the enemy caster was at the time the fireball was cast... but he'd be dumb to stay there, so it's of relatively limited use in the end.
Sniping rules only apply to your stealth check AFAIK, not to perception rolls. As such they probably should not apply at all in the case of a non-silenced spell, since you're talking loudly for seconds on end and completely obliterating any notion of stealth. It's not like popping out for moment and loosing an arrow.
I would disagree with this completely. You are talking, but not loudly, and for a few second.
There are not hard and fast rules, so I apply the rules I think apply best.