Is there anyone we know for certain is in hell?
The short answer
Satan and the fallen angels certainly are, or will be on the last day. Scripture implies that some human beings will not be saved, but we are not given to know who. The Church prays for all the dead and hopes for the salvation of each person.
The fallen angels
Satan and the fallen angels are certainly in hell, or will be on the last day (CCC 392-393).
Human beings
Scripture implies that some will not be saved (Matthew 7:13-14), but it has not been given to us in this life to know who has failed to find salvation.
The Church's hope
In union with God, who wills everyone to be saved (1 Timothy 2:3-4), the Church prays for all who have died and hopes for the salvation of each one (CCC 1821).