If God is the only one that can indicate whether or not one is on the right track, then how is it that you know someone cannot be truly righteousness? Wouldn’t God be the only one that could know that?
Because it's one of the truths in the Bible which is emphasized again and again.
Here's a couple of samples:
From the Old Testament: "all our righteous acts are like filthy rags and shrivel up like a leap and like the wind our sins sweep us away" Is 64:6
From the New Testament "just as sin entered the world through one man, and death through sin and in this way death came to all men, because all men sinned" Rom 5:12
The sin of man vs. the righteousness of God is one of the major themes of the entire Bible. You can't miss it if you are sincere in your Bible study (which the Holy Spirit within you will prompt you to be).
“leap” = “leaf”