The Bible says, "False prophets are people who prophesy or say things in Gods name - as if it were His very Words. We can know them by testing what they are saying or teaching, if these things actually take place or comes true. If what they say does not come true then such people are the false prophets as determined by God " - (Deuteronomy 18:20-22).



Allow me to explain with an example.
Those preachers who say things like,  'Christ' blood on the Cross ALONE cleanses you from all sin,' and later people find out who believe in this gospel that nothing really has changed for them, they still have sin; then this is clear proof that what these preachers have said were utter lies, they have proven to be the false prophets.  What they have said in the Name of the Lord has not taken place, their gospel could not change the sinner into a sinless saint.