To truly be a Christian, you must not only believe God/Jesus exists but also accept His gift of salvation and accept Him into your heart to essentially "take the wheel." True Christians ought to know where they're going and not be afraid of death, because they know they will be with their Savior. True Christians ought to live for Christ, by showing His love and telling others about the Good News - that He died to take the punishment of our sins so that we can live forever with Him.
True Christians, however, usually struggle with doubt. Because they belong to the Lord, they are often attacked spiritually by Satan, trying to discourage them. One common doubt Satan plants in people's minds is that one's "not good enough," or "too far gone" to be saved. Even those who have been Christians for decades still struggle.
Jesus did not promise that we would have easy lives, rather He said the world would hate us. John 15:18-20 says, "If the world hates you, keep in mind that it hated me first. If you belonged to the world, it would love you as its own. As it is, you do not belong to the world, but I have chosen you out of the world...If they persecuted me, they will persecute you also." This is clearly seen in the amount of Christians around the world that are persecuted for their faith. However, they refuse to convert/stop meeting/stop spreading the Gospel. This resilience and willingness to lay down their life sets them apart because most people love their lives. You know why they can withstand being tortured, even killed for their beliefs? Because God gives them strength, and they know that what awaits them on the other side is better than anything here on this earth. That is a sign of a true Christian.
True Christians should be obvious in word and in deed. They should be joyful, even in difficult circumstances. They should be kind, even to the cruelest people. They should be different - a good, visible different that makes people say, "I want what they have!"
However, some people who profess to be Christians don't really have God in their lives. Some have accepted Him, but they struggle and aren't at a point where they understand how to live for God yet. Some people become Christians but fall away during difficult times. This is what causes people to misunderstand what true, strong Christians are really like.
True Christians should be easy to spot. They should be like "little Christs" - in fact, that is the meaning of the term "Christian". However, while there are indeed certain things that all Christians must do to be saved, I would like to point out that no Christian is the same. We're all at different points in our spiritual walk. We all struggle with different things. We all screw up. Some of us have different standards (more or less). We aren't all going to be the absolute epitome of Christianity.
But over all, true Christians are united in Christ. Because no matter how different our lives are, or the struggles we go through in our physical and spiritual lives, what really makes a Christian a true Christian is that God is in their lives and He has changed them from the inside out.