Is there any canonical reaction that the average NPC (with no ability to identify what spell is being cast) to someone who starts casting a spell in a city or town in Golarion?
Something mundane like Prestidigitation to clean one's boots before entering a building, or Message to speak with companions over the loud background noise of a tavern, or even Identify or Detect Magic to ascertain the quality of goods being sold?
Would a suspicious or violent reaction be justified?
Personally I would say no, general magic use is commonplace in most places and it should not arouse suspicion unless you happen to be coinciding with someone else casting a spell that would be determined to be illegal (say someone noticing a charm, or seeing an item floating away etc..) Possible exceptions to the very low/no magic parts of Golarion
Honestly I always assume that casting spells in polite society that had obvious gestures and words would make people nervous. I could never understand Charm Person for example being Verbal and Somatic components and no one ever freaks out taht you just cast a spell on them or worse their buddy. It would be one thing if its a person by themselves while no one is looking, but if 2 guards stop you and you charm one, why is the other not freaking out or asking what you just did? Seems odd to me.
Honestly I always assume that casting spells in polite society that had obvious gestures and words would make people nervous. I could never understand Charm Person for example being Verbal and Somatic components and no one ever freaks out taht you just cast a spell on them or worse their buddy. It would be one thing if its a person by themselves while no one is looking, but if 2 guards stop you and you charm one, why is the other not freaking out or asking what you just did? Seems odd to me.
Oh absolutely, I'd say in the case of 2 guards and you cast charm on one, the uncharmed would almost certainly piece together his buddy's abnormal behavior with your spell-casting a nearby adept or wizard also may succeed a spellcraft and call him out, and if so expect an immediate shout for backup as mind effect/controls are not taken kindly in a normal society. I'm pretty sure that they would freak out if you were doing something clearly illegal with your magic, but not simply for carrying the spells. Same reason why the guard wouldn't freak out watching a guy with 2 swords and a crossbow strapped to himself, walk into a shop, unless he hears screaming. Every town has a good handful of wizards walking around using spells for everything from cleaning their boots, enough that people can't afford to freak out every time someone is casting a spell.
For me it depends on where exactly you are and what you are doing.
Standing outside a building while others are knocking mud off and you are looking at your shoes casting? Probably no big deal (though with as long as prestidigitation lasts I usually simply keep it cast all day).
Outside of a building with guards and casting? They are probably going to have issues with that.
At a seance or 'magic fair' or something? Probably not a second look provided the effects are what are expected (casting snapdragon fireworks at a fair is probably okay, casting lightning bolt is probably not).
Also it's partly how you prepare the people around yourself for it, "Hey guys It's a bit dark in here I'm going to cast dancing lights to make it easier to see." Will probably go a long ways to not freaking everyone in the tavern out when you start casting.