Pride is mother of all sins, Augustine said. But it is also the sneakiest. In short, a sneaky mother. So how do you know if you are proud? Jonathan Edwards clues us in:

“But though spiritual pride be so subtle and secret an iniquity, and commonly appears under a pretext of great humility; yet there are two things by which it may (perhaps universally and surely) be discovered and distinguished.

The first is this; he that is under the prevalence of this distemper, is apt to think highly of his attainments in religion, as comparing himself with others. It is natural for him to fall into that thought of himself, that he is an eminent saint, that he is very high among the sainsts, and has distinguishingly good and great experiences. That is the secret language of his heart, Luke xviii. 11. “God, I thank thee that I am not as other men.” And Isa. lxv. 5. “I am holier than thou.” …

But he whose heart is under the power of Christian humility, is of a contrary disposition. If the Scritpures are at all to be relied on, such an one is apt to think his attainments in religion to be comparatively mean, and to esteem himself low among the saints, and one of the least of saints.  …

Secondly, another infallible sign of spiritual pride, is persons being apt to think highly of their humility. False experiences are commonly attended with a counterfeit humility. And it is the very nature of a counterfeit humility, to be highly conceited of itself. False religious affections have generally a tendency, especially when raised to a great height, to make persons think that their humility is great, and accordingly to take much notice of their great attainments in this repect, and admire them.”

—The Religious Affections