I personally really question myself and hesitate. Knowing one's own motives can be difficult enough, much less presuming to know another's. Many mistakes can and have been made in attempting to make those judgments - and can become just that - judging.
"Humility is defined as, "A quality by which a person considering his own defects has a humble opinion of himself and willingly submits himself to God and to others for God's sake."
Humility is said to be a fit recipient of grace; according to the words of St. James, "God opposes the proud but gives grace to the humble" (Proverbs 3:34, 1 Peter 5:5, James 4:6 NIV).
"True humility" is distinctly different from "false humility" which consists of deprecating one's own sanctity, gifts, talents, and accomplishments for the sake of receiving praise or adulation from others, as personified by Uriah Heep. In this context legitimate humility comprises the following behaviors and attitudes:
Submitting to God and legitimate authority
Recognizing virtues and talents that others possess, particularly those that surpass one's own, and giving due honor and, when required, obedience
Recognizing the limits of one's talents, ability, or authority." wikipedia (that bastion of truth)
I believe sometimes a person can be working on humbling themselves before God, battling the flesh etc. in an honest manner (not attempting to receive praise and adulation) and that could possibly be perceived as false humility. Or even vice-versa, i.e., someone may really know their stuff, and be wrongly perceived as prideful.
I guess as with most things, it can become complicated.