Christians: Do you believe someone when they tell you that they are a Christian?

The Bible talks about those who will try to fool in the name of the Lord. It also talks about there will be some who will be lukewarm in the name of the Lord and there will be some who will be on fire. I think that if someone is truly in the spirit and has completely turned themselves over to the Lord then you won't have to question if they are a christian or not you will see it in everything that they say or do.