How do you determine allies & foes?

[Velkyn]

Cross-posting this in general discussion as I don't think RAW weighs in:

During play, our group could not agree on whether Blistering Invective would affect undetected opponents in the area. I believe the two sides of our debate were:

• No: The caster chooses who their enemies are when casting spells that affect "enemies" (and likewise for allies). Because the caster was unaware of these creatures when casting Blistering Invective, they could not be harmed.

• Yes: The laws of magic determine enemies and allies, thereby making the distinction objective. Any creature wishing you harm is an enemy, while all others are allies.
  

Dazzling Display would be a non-magical example of this issue. I do not believe the rules are clear as to whether an invisible/stealthed individual would be considered a foe.

How do other groups handle these situations?

[wraithstrike]

This spell calls out enemies that hear it. Even if the caster does not know certain creatures are there the creatures know the caster is an enemy so they shouldstill be affected. As for your general question there really is no rule for it. Most of the time it just comes down to a matter of perception.