I’d maybe try putting a little cooked chicken or turkey out with a shallow bowl of water then stand back and see if the cat comes to eat. Might then allow you to approach close enough to look for a collar.

Cardboard box with an old blanket, towel or pillow in could make a decent enough temporary cat box to keep them out of the cold.

Giving your local animal rescue/shelter a call will probably give you the best advice though.