Quote Originally Posted by Bendraesar View Post
But who defines feeling a hostile force as unnatural? The example NPC is a guard, not a merchant. A guard in most cases has to deal with hostile criminals, monsters, etc., putting their life on the line to deal with threats to the city they're guarding. So they'd be pretty used to feeling hostile intent, mystical or otherwise, this sudden feeling is nothing new. (Now if you want to say the guard has lead an easy career... this is just a DM purposely trying to screw the caster).
Well then if this is nothing new, then they should already know what a spell being cast on them feels like. They might even raise an alarm "Someone just cast a spell on me!" and be even MORE suspicious than the one who led an easy career.

Like I said, I doubt we'll get an agreement on what the hostile force actually feels like. I'm using headache because that was the first one given in the thread and can be conveyed as the DM not actively playing against the PCs.
This is true, only the DM would really be able to define what it feels like. However I still like to imagine it as quite distinctive, otherwise the description would not be so vague.

Assuming they knew they just resisted a spell.
Well the text can be interpreted that way.. of course it can also be the reverse, it's really too vague, but I would think the fact they put anything there at all would imply they are supposed to know something at the very least.