It's not always possible to tell, without bringing the animal inside and
getting acquainted. Some of the sweetest cats "feral" we've rescued were
discovered to be tame after they had been caged for a while (the two I
have in mind were both injured and required cage rest). Other guys we
thought were abandoned pets let us know that they were born to be wild.

In your place, I would trap him, take him straight to the vet for checkup,
shots and surgery and then isolate him from your cats for at least 2 weeks
(to rule out impending illness/contagion) and wait and see. With just a
little time a patience, he may own up to being tame or just may decide to
at least give domestication a chance. Even if he is feral, he may be a
sweetie with your indoor cats and will make a nice under-the-bed house
feral.

Be sure to combo-test him for FIV/FeLV; toms are likely carriers. If he
tests positive, have a talk with your vet about how to proceed.

There are lots of articles out there on socializing a cat. Mine is
"Taming the Tiger," posted to the Campus Cats website.

Good luck with this guy. Good for you, giving him a chance.

Sharon Talbert
Friends of Campus Cats
www.campuscats.org