how to find items in Photos that are "on This Mac Only"?

I have 3000+ items in my Photos library. At the very bottom of my pictures stream (the most recent stuff), it shows: 3048 Photos, 136 Videos (1 item on This Mac Only).

How do I identify the 1 item that is on my Mac Only?

I use iCloud syncing and as far as I can tell everything is setup correctly. When I take a picture on my iphone, it automatically appears in Photos on my Mac. When I delete or edit a photo in Photos on my Mac, the deleting or editing is reflected in Photos on my iPhone.

But how do I identify that 1 item that somehow got into my Mac Photos library without getting sync'd to cloud?

Help please? Thanks!