Do you know if a person is a Christian or not?We cannot know for certain whether a person is or is not a true Christian (only God can makes such a determination). However, the Bible describes the nature of a person who is indwelt by the Holy Spirit (which happens at the point a person accept Jesus as Lord and Savior):
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law. (Galatians 5:22-23)
In addition, the true believer does what is right and demonstrates love toward others. Even non-believers can readily see who the true Christians are. Ultimately, the behavior of those who claim to be Christians but practice evil should not be a consideration in determining if the claims of Christ are true or not.
Yes, "Christians" have committed atrocities against other religious and non-religious people. However, atheists have committed far more atrocities than all religious groups combined. Even so, the key factor in these atrocities has been totalitarian power, rather than religion, which has resulted in these hundreds of millions of murders. The Bible says that people are evil, but that they can become transformed through the power of the Holy Spirit to live lives of love, joy and peace.