Apple - How to find out process owner of unidentified windows?

• Download and install Xcode.
• When installed open Xcode.
• In the menubar go to Xcode > open Developer tool > Accessibility inspector
• In the Access..Inspector menu go to Inspection > Enable point to inspect
• Click on anything to see the result

It does not give you the process name, but it does give you the parent's application name

I was also trying to figure this out and ended up using the solution described here. It's a Python script which lists all the processes with all the windows that belong to them and their coordinates on the screen. So it is not as easy to use as the Process Explorer on Windows, but it can get you the answer your are looking for. It helps if you can move the window to a place on your desktop with easily recognizable coordinates.