There are, no doubt, a variety of ways that people in the world might know that you are a Christian. For one, you could just tell them. Walk up and tell individuals, “Hello, I’m a Christian.” (Nothing wrong with that, and our world might be a better place as a result.) Or, someone else 
could tell people for you. “Did you know that he is a Christian? Just thought you should know.” A more appropriate way, no doubt, is to “let your light so shine before men, that they may see your good works” (Matt. 5:16). Let others see you living the Christian life!
With less than twenty hours left in His own life, Jesus gave a guaranteed way for ALL to know who the Christians are. “By this all will know that you are My disciples, if you have love for one another” (John 13:35). Christian love (Greek word, agape) distinguishes Christians from everyone else in the world; it is a quality that is impossible (even for Satan) to fake or fabricate! Christian love is action-oriented (cf. 1 John 3:18)! When you act like Christ, with unselfish and unconditional love (Phil. 2:3-5), the world will know that you belong to Christ! Guaranteed!