Christians have a fish on the back of their car. Non-Christians have a fish with legs on the back of their car.
We do not know who is and who is not saved, if that is what you are asking. One person may be undergoing the discipline of a Father (God disciplines Christians, not non-Christians) for a lifestyle we may assume to indicate a life without Christ.
We are not called to know who is and is not a Christian. We are called to live holy lives, which includes judging people by their fruit. Regardless as to the person's salvation we recognize what is of the Light and what is of the darkness, and we make determinations based on the fruit.