How can I check if MySQL databases are still in use? #Get out

I have a cleaning situation here, the old programmer did not clear his databases and users due to use. Although some of the databases are still used by external sites (on other ftp servers), some of them are outdated and just clutter up the system.

My question is, how can I determine which databases (and users) are still in use by other websites? ( without checking every website that has ever been created, no matter where it is)

I need to make sure the MySQL database (& user) is no longer used by any site, so I can safely delete it to clean up the system.

ps: It could also be that the database is still in use but does not do any INSERT or UPDATE, but only SELECTs the data to load the website.

pps: I cannot (temporarily) deactivate / delete the databases (& users) because this would cause clients to lose revenue, customers, search rankings, etc. and in the end it will cost us / me.