i know a man that is a member of the Jehovah witness, who does not agree with them about everything they teach. he professes to be saved by Jesus, and i can see the love of God in his life.
the Truth is that none of us can tell if someone else is really a Christian, because only God can see the heart; we can't even know this about ourselves - we must simply believe that we are reborn in Christ with no evidence to support this other than God's revealed written word; that's what True faith is - belief in God's word alone as the exclusive authority of Truth.
1Sa 16:7 KJV
(7) But the LORD said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart.
Mat 7:15-23 KJV
(15) Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.
(16) Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
(17) Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
(18) A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
(19) Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
(20) Wherefore by their fruits ye shall know them.
(21) Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
(22) Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
(23) And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.