(Proverbs 23:7 KJV) For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.
We all have a secret person, the person of the “heart” (3820 second usage in KJV). In some people it is clear who that is. We have a saying about people who wear their heart on their sleeve. Others make it very hard to know what is really going on inside. This warns us to be careful in our assumptions. When it comes to people what you see is not what you get.
Part of that is because people are complex. We have many little pockets in our personality and we don’t share them all. We are also constantly changing. It is possible that what was true about you last year isn’t true this year. The changes might even be daily.
This is referring more to the person who deliberately deceives. The KJV talks about the “heart” (5315, first usage). The NASB says “within himself”. The world is usually translated “soul”. Beware of people who are deceptive of their real motivations.
I think there is also a direction to us to make sure that our souls are in the right condition so that our actions are loving.
So? You can’t read the other guys heart but you can do something about your own. Pray for honesty with yourself and strength to act on it.