[Animayhem]

It depends on where you are. If in wilderness even if a person in stealth they can be heard of twig snaps but you have to take your character's race into conicderation. For instance if you are elf or half elf you may be able to sense people easier. As mentioned paladins can sense evil and also as also mentioned the person in steath may not know they have been spotted.

In the city it would be much harder as there is noise so you would have to be trained to do so.

Which brings up another question. The game mechanics have certain spells and ioun stones emit sounds while in play, so if someone has spells as such and walk near you one could notice them. So in that case would it be fair that the person is identified especially if the who detected them is of the arcane for instance.