One of the easiest ways to tell the difference is by examining the direction of thoughts. If you are happy to be humble, then that isn’t prideful because all virtues are good and desirable and we’re suppose to be happy with developing them. I don’t know if anything causes more avoidable pain and grief in the world than pride. It’s as if life were a series of large ocean waves, and instead of easily diving low & into the wave to pass through them, a person continually tries to overpower them and ends up always exhausted and agitated and hurt. Getting rid of pride is one of the most or the most happy thing anybody could ever do.
But, if thoughts start going in the direction of making comparisons to other people as a way of gaining satisfaction, then it is pride in humility and that should be patiently removed by redirecting thoughts towards happiness and gratitude. Daily prayer should always include saying thank you to Jesus.
I agree that C.S. Lewis is a pretty good author on the subject.