"How do we know???" I'm sure I have commented on this before but it is such a good passage I wanted to highlight it again.

When we accept Jesus we get the Holy Spirit and the Holy Spirit speaks directly to us.  But this passage makes clear that there are other spirits out there who will try to influence us as well.  So don't just assume that when a spirit talks to you that it is the "Holy Spirit".  Remember Satan is the prince of deception. 

So how do we tell?  We are told to test the spirits to see if they are of God. 

1 John 4:1-3(NKJV)

Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. 

By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, 

and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world.

How do we do that?  We test to see if they confess that Jesus came in the flesh.  Where I think this is especially useful is in evaluating religious theology.  There are many different branches of Christianity but they all have the commonality of belief in the birth, life, death, and resurrection of Jesus.  Without this common belief they aren't really Christians are they, because the very essence of Christianity is the belief in the birth, life, death, and resurrection of Christ.

There have been many cults and false religions that seem to be "Christian" but when you get down to the basics, they don't really believe in Jesus.

So for me at least, this gives a quick down and dirty way to analyze knew "beliefs" within the Christian world.  If it passes this first test then I can explore it further if I want to or fee led to.  If it doesn't pass this first test then God has just saved me from wasting a bunch of time. 

Pretty Cool!