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 the 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 which specific section should I focus on?