I would ask... but I would try my best to find owner before I took to vet. I would have scanned for a chip etc. Of course if it was a life threatening situation I would take to the vet.
Of course, like many others have said, I would pay a vet bill if someone took one of my guys to the vet. Life happens, I would like to think mine would not get lost ..and they are chipped.. so I sure hope I dont have that situation