False Prophets

What does the verse "know them by their fruits" mean?  What is this referring to?  Kayne and Luke discuss how to detect the spoken word in someone.  How does the power of discernment protect you from people that may be trying to persuade you into a belief system?  Even if the person is well intended, does that mean harm can be done?  Is the road to hell really paved with good intentions?  What exactly is a good intention when applied to a spiritually unconscious person?