Question: What does Proverbs 27:19 mean?
The whole of the subject in this Chapter is the caprice of mankind (male and female)!
In the Bible the heart is considered the seat of life or strength. Hence, it means mind, soul, spirit, or one's entire emotional nature and understanding.
In Jeremiah, God laments this condition, which is a result of Eden’s fall! Jeremiah 17:9 says, “The heart is deceitful above all things, and desperately wicked: who can know it?”
The answer is that only Almighty God can look upon the heart of mankind; and His statement regarding King David is expressive of this:
The Bible calls David “a man after God's own heart” twice. The first time was by Samuel who anointed him as backslidden King Saul's successor, “But now your kingdom shall not continue. The Lord has sought for Himself a man after His own heart” (1Sam. 13:14, NKJV).
God repeated this in 1Sam.16:7!
This shows that God in His Power knows the future of each person!
So; the answer to your query is encompassed in the above; a person is known by what they reflect!