Is there a way to determine what application/process is supposedly using a file/folder? There is a stubborn folder in my trash bin that apparently cannot be deleted because "something is using it". I know how to force empty the trash bin, but rather than doing it, I'd like to know exactly who is using this file.

Is there a way to determine what application/process is supposedly using a file/folder?

Mac OSX Yosemite.
Yes, the lsof command will show you that.
Open a termin and type "lsof [folder name]" and the command and process id should be listed.