What does it mean that we will judge angels, and when did the angels fall?

I would not conclude that not all the angels fell in the beginning. Paul's statement in 1 Corinthians 6 that we shall judge angels is a one-time statement about a one-time event, so we cannot make too much of the details. But we know from Revelation that believers will reign with Christ as kings and priests and sit with him in heavenly places, and that the apostles will sit on twelve thrones judging the twelve tribes. As Catholics we also believe in the particular judgment, which happens the moment you die, when you are individually judged and sent to heaven, hell, or purgatory. Then there is the general judgment, when everyone is gathered and Christ, with the saints, officially confirms and makes public the judgment already rendered. The same is true of the angels: they have already had their judgment, and at the general judgment we will take part in confirming it. That judgment of the evil angels was rendered in the Garden of Eden, when the devil and his angels tried to tempt Adam and Eve to disobey God; that is when their sin occurred. Nothing occurred before Genesis 1; there was only God, who created the heavens and the earth.