How can I identify what application is using a given file?

Question

I'm trying to delete some folder but this error is preventing from doing it:

Cannot delete [FileName]: It is being used by another person or program.

Close any programs that might be using the file and try again.

How can I identify what application is using the file?

I want to end the application using a Task Manager, but I didn't know what application is using it.

The folder is not shared and my user account is an administrator.