Let's take this Bible verse at face value (not digging for deeper spiritual meaning which God's Word always has) and look at it's face value first and foremost. We read that those who judge others are ironically guilty of the very thing they judge others for! So, if you want to really know what's deep in someone's heart, pay close attention to the words they speak and the judgements they make against others.
Pay very close attention indeed.
~dkg