"Christian" is an Extremely wide category. this maybe slightly rough shot, but if you want to know whether a certain faith is part of the Christian tradition, look at the text they use. if it 1) says Bilble on it, and 2) is broken into The Old and New Testament (lacking a section entitled Qur'an), then you are probably dealing with a Christian. Like I said this maybe a rather obtuse approach, but it allows someone with no experience in Christian theology a chance to grasp the idea of what is a Christian. 
 
In the interest of full faith and disclosure I am not a Christian, an agnostic or atheist.