It’s been said that we must not judge others, as we have not walked in their shoes and can not see their hearts. In other words, our lack of knowledge about the motivations and experiences of other individuals makes it impossible for us to know whether or not they are holy or evil people, and to what degree. At least, that’s the way I’ve always interpreted this teaching.
My question is, is it possible to know this even about ourselves? If our actions are affected both by our free will and also by our prior experiences, unique personality, and psychology, is it possible for us to figure out the content of our own hearts, and to understand ourselves? If so, to what extent is this possible? Can we be precise or are we limited to ballpark estimations? And furthermore, is constant examination of our character something we ought to be doing in the first place? C.S. Lewis, at one point, suggested that evaluating our own holiness should not be something we put too much emphasis on.
We do not judge people but we do judge behaviors. So if I see a thief, a drunkard, or whatever, I do not label him a useless, inferior man worth nothing. I condemn stealing, I condemn abusing alcohol. But I do not judge the man, as in: I do not condemn the man. Chances are that if I had been in his shoes, I’d be 10 times more of a thief and drunkard than he is.
No, it is not immediately or entirely possible to figure out our own hearts. Which is why just as Scripture tells us to not judge others, it also says (in the words of st. Paul): “
I do not even judge myself, for even if I am not conscious of any fault, it is the Lord who judges me”. What Paul says here is exactly what you are saying! There is no way I can know my heart entirely - only God can do so. So the psalmist prays (psalm 19): “
Who can discern their own errors? Forgive my hidden faults.” and (psalm 130): “
If you, O Lord, will mark iniquities: Lord, who shall stand it? But there is forgiveness with you, that you may be feared.” To the prophet Samuel, God said: “
The Lord does not look at the things people look at. People look at the outward appearance, but the Lord looks at the heart.”
Sainthood - as far as the saints teach us - is manifested in a man’s awareness of his sinfulness. The bottom line is he who says: “I have no sin”, for this is gravely erroneous - all have sinned. The top of the line is Christ, who “became sin” for our forgiveness. Indeed Paul is not ashamed to say: “
Here is a trustworthy saying that deserves full acceptance: Christ Jesus came into the world to save sinners–of whom I am the worst.” It would not only be impossible to measure our holiness or evil - we can only grow in the awareness of our sinfulness. This is an intellectual process which overcomes passions. It takes time, and silence, and introspection.
And it is also rooted on meditating on God’s perfection. It becomes then quite evident - at least, after a while - that no matter how perfect our actions and intentions may seem to us, they still fall quite short of God’s goodness. For us, meditation on Christ Crucified is - no pun intended - crucial to attain this awareness: for we believe it was our sins that held Him on the cross, not the nails. We live, thus, with a “penitent heart”, a “repentant heart”, with “contrition”, and, following Christ’s words, we start serving Him in the least of our brothers, realizing that whatever we do (or not do) to the least of our brothers, we are doing (or denying) to God Himself. This is the “folly” of Christianity - to see God in the lowest man, in the man that we despise most, in the eyes of the one person we wish we had never known. For it is quite easy to see God in the eyes of a child, or a spouse, or a parent, or a best friend. It is quite easy to see God in the eyes of the firefighter that rescues us from death. Difficult is to see God in the eyes of the stranger, of the beggar, of the guy who hates me, who makes my life hard, whom I despise more than anything else. Whom I’d rather ignore.
I don’t want to switch to preacher mode, since I’m no preacher myself. I just mean to clarify that: indeed we ought to constantly, persistently try to enter the “inner mansions” of our heart, examining our character and our conscience, reflecting on our imperfections, on our flaws, on our disordered inclinations and desires, and aiming not just at being good people but aiming at becoming - again, more Christian folly - as perfect as God the heavenly Father is perfect. Because this is our belief: that as humans, we are made “in God’s image and likeness.” In this way, we keep purifying ourselves, never satisfied of our “holiness”, and yet never disorderly distressed about our sinfulness (since we are, after all, mere humans, inherently weak and weakened in our very nature by pride, vanity, and lust). We will always find that we could be serving God more. There is always something more we could do for the sake of goodness, of justice, of mercy, of truth. But we need introspection, constant struggle with ourselves, stepping out of our comfort zones, and self-sacrifical mindset (“sacrifice” in Latin means “to render holy”).
Mysticism is to enter the inner sanctuary where God dwells, and as a consequence, forget more and more oneself and see Him more and more in His creation, and serve Him more and more in His creatures. Clearly our state of life does not always allow for a fully contemplative life, and oftentimes we only have little time for introspection… However, going about our duties, and also about our little joys and miseries, with a contemplative heart and a self-sacrifical mindset will still give us opportunities to try ourselves 
For instance: suppose I am driving and someone does something I don’t like and I want to say something uncharitable…here’s a perfect occasion to look into my conscience and make a mental note of my little failure at charity 