I read a story in one of Watchman Nee's books where he ran across another Jesus. He was going door to door, evangelizing.  He asked this woman if she knew Jesus and she enthusiastically said yes.  This was surprising to him, since no one else had evangelized in this area.  After further pursuing the issue, it turned out one of her family spirits was named "Jesus".  After sharing the real Jesus to the woman, she became a Christian.

In my Episcopal region (called a diocese) there was a meeting of priests, where one priest from Texas shared the simple gospel plan of salvation and offered an altar call to the priests.
Suddenly a woman priest jumped up and yelled, "My Jesus is not your Jesus! My Jesus is just as valid as your Jesus!  You have no right to push your Jesus on people!"  What a show-stopper.  I wish I could say she was defrocked on the spot and kicked out of the church.  I cannot.  I have since left.
A man I know who had visited her church previously had gotten demonized by receiving communion from her. I had to clean him up. Evidently, she had redefined all the other sacred words, so the effect of her prayers before the altar were profane.  (Sorry, priests, but this really happened.)

The Muslims and Gnostics believe in a "Jesus/Christ" combination.  Their "Jesus" was a human who was infested with a "holy" spirit named "Christ".  This "Christ" did all the teaching through its host, "Jesus."  This "Christ" pulled out of the poor sap before he was crucified, but "Jesus" somehow survived.  No explanation is given for why his blood had separated by the time the soldier speared him.

The Hindus gladly accept Jesus as their Lord -- an Ascended Master who escaped Karma and reincarnation.  Thus they need to be asked if Jesus Christ of Nazareth is their only savior and master.  A more polite way to smoke them out is to ask them to describe their Jesus. 

I have read that Satanists who infiltrate a church will redefine Lucifer to Jesus, so they can talk to all the Christians about how wonderful "Jesus" is. But if they were asked if Jesus Christ of Nazareth is their savior and master, they can't say "yes", or their master will kill them.

I visited a New Age expo several years ago.  At one booth was a woman who called herself a "reverend", had a beat-up lump of a Bible at hand, and was hovering over her crystal ball.  I generally despise counterfeit Christians.  So I asked her if Jesus Christ of Nazareth is her lord, savior and master.  She was thrown back in her chair and screamed, "No!"  With that I walked away.  If she had accidentally said, "Yes," I would have offered her protection through the real Jesus -- if she became a Christian.

In Jesus' day, there were a lot of men named Jesus.  The Jewish version of "Jesus" is "Joshua", like the fellow in the Old Testament.  Thus it was not surprising that there were other men named "Jesus", even some who called themselves the messiah. Times were hard and the people were looking for a messiah.  (Matt. 24:5)

The Apostle Paul was concerned about this problem. "But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ. For if he who comes preaches another Jesus whom we have not preached, or if you receive a different spirit which you have not received, or a different gospel which you have not accepted--you may well put up with it!" -- 2 Cor. 11:3,4

Even today we can be tricked by people who preach another Jesus. (2 Pet. 2:1)  Only Jesus Christ of Nazareth points to the true, living God. Thus it is important to describe which Jesus our prophets, teachers and leaders are referring to.

A minimum confession to test for false teachers and pastors would be "Jesus Christ of Nazareth is my Lord, savior and master." If your pastor or teacher can't look you in the eye and make this confession, separate your self from them. Don't even eat with such a one. (1 Cor. 5:11)

Candidates for leadership should be asked, "Describe to us your Jesus.  Describe to us your god.  Describe to us your Holy Spirit."  If what they say is unambiguously Christian and unambiguously their beliefs, you have a Christian.  Otherwise, something is wrong.

Another way to tell a true Christian (although they can be babies, carnal or backslidden) is to maneuver them into exercising deliverance authority.  All Christians can do that; it's not a gift only a few have.  No one but Christians can do that.  The best anyone else can do is to negotiate a demon-swap for something worse. 

In counseling, the way to tell a demon in someone from a split personality is the demon can't say, "Jesus Christ has come in the flesh."  (1 John 4:1-3)