There isn't a good way to do this. It isn't tracked per se, so you can't be sure when someone last used a database.
If you suspect no one is using a database, set it to auto close, and if it is accessed, an open message will appear in the log. however backups may open the database, so I'm not sure this is that easy.
You can set up a trace to watch for queries in the database, or you can set it offline, and wait until someone complains.