how to check whether Oracle database is accessed or not?

I am an Oracle DBA. Application owner is asking me when was the last time Oracle database was accessed? How do I find information about the DB usage timeline ? I mean, i want to check whether it is being used by the application or not ?

Unfortunately Application owner has no idea whether her application is used by the users or not.

From DBA's side which data dictionary should i look into ? Can ASH help ? if yes, then when specific section should I focus on ?

Regards, Pratik