You will know them by their fruits. This verse is often misquoted to mean we can judge who is a true believer by evaluating their life. Jesus says here that false prophets (false teachers) will always teach error and live selfish and self-serving lives (2 Pet. 2:1-3). Like wolves, they feed off of the sheep. See Ezekiel 34. For outward fruits of our faith see Galatians 5:22-23.