To find out if a DB is not used anymore set it offline and wait for someone to complain 🙂

If noone complains, then you have one less DB to worry about.

Seriously though, one way to know if a DB is not in use is to take a trace (only audit login/logout) and see if there is any login activity other than the maintenance jobs.

Let it run a few days to be sure (week-end included) then if there are no activity do a full backup, zip it, archive it and drop the DB after leaving it offline for some time.