Is it obligatory for every Muslim to believe that a person, who is a confirmed to be a kafir, is a kafir?

Is it obligatory for every Muslim to believe that a person, who is confirmed to be a kafir, is a kafir? I don't know if my neighbours are kafir or not. But we are of a different race and most of the people of my neighbour's race are kafirs. Some of my neighbours also own dogs. If I don't regard them as disbelievers, does that make me a disbeliever? Jazakallah khayran.