does it smell bad? sometimes you can tell from the piss smell if its a stray. you can also check for fleas yourself.
even really scrony cats might not be strays could just be really badly looked after or recent rescues. either way, if it's skinny and doesn't seem to be in pain then feeding it healthy cat food (not cheap, salty wet food!) will be fine.
if you're concerned, you can borrow a cat box and take it to the vet to see if it it's chipped - they won't charge.