J. I Packer answers this way:

The true answer is that it cannot be measured. Growth in grace is a mystery of grace, which it is beyond us to monitor in either ourselves or others. Observables, like zeal, knowledge, self-image, and behavior patterns, are ambiguous: they may be carnal at bottom, though spiritual-looking on the surface. The heart of growth is growth in the heart, which only God can search and know.
However, something of our spiritual stature may be discerned by our responses to what we call crises of decisions and Scripture calls temptations. Those who deal with crises, or temptations, better than they once did show that they have grown in grace in the interim.

You can read the entire article:  Crises of Faith Are Yardsticks for Growth