This is only a guess !!
Maybe by at least Two of us or more as per according to scripture. Takes two witnesses to convict.
And maybe the angels who followed satan is the angels we will judge.
But this is just guessing !!
i think the statement in 1cor.6:3 is a question(do you knowthat we shall judge angels?) 2peter 2:4 for if GOD did not spare the angels who sinned,but cast them down to hell and delivered them into chains of darkness ,to be reserved for judgement;