If you open the
Google Photos app and go to
Menu > Device folders, that will
only show photos which are currently stored on your device.
From the main
Photos view, you can open a photo and look for a little "backup to cloud" button at the top right if a photo hasn't already been backed up.
I feel like there used to be a better way to tell at a glance, but these are the only ways I was able to check just a moment ago.
(PS - I moved your thread to the
Google Photos channel in case another Photos user knows a better way.)