It's easy around here to distinguish the 'outside cats' vs the feral ones (there's a feral colony not even a block down, plus maaany others in town). The tough part is, like you said, if it's an outside cat or a cat that got outside. :/ The rare times that one follows me, I'll give it a treat or two (or a can of food if it's uber skinny) but otherwise I just ignore them or i'd have fifty damn cats by now, lol. Post a few flyers or just house it and see if anyone posts 'lost' flyers?