What are the telltale signs that a cat has no owner?

Some Context on the cat I'm worried about in particular

I have a cat who jumps through my window into my home and just walks around. He runs whenever someone walks, but he has no problems if there are awake humans (There are no animals in my home, however there are several dogs around)

Recently I started leaving him some food or even feeding him by throwing some food out the window (He refuses to come close). He does seem to be hungry all the time, however he does have a necklace and doesn't seem to be weak (He has to do quite a big jump to get to my windows)

Other than that I know nothing of the cat, except that he panicked the one time I accidentally closed the windows with him inside. I'm unsure if the cat's scamming me for free food or if he's abandoned

Due to that, my question. How can I figure out if a cat has no owner, and if you would so kind (although I do understand it might be too localized), how can I tell if this particular (seemingly fine except for the recent approximation he has made towards my house) cat has an owner?