Help Cleaning up photos from phone after sync'd to Google photos

[electricpete]

Let's say you want to keep photos for eternity on Google photos, but you'd like to remove them from your phone.

As discussed here, you can't clean them off the phone using file explorer or gallery apps...that'd delete them from Google as well.

So instead you have to go into the Google photos App on Android, select photo of interest, and select "delete device copy"

That's fine, but I've got thousands and thousands of photos in Google photos (most from my pc). How do I navigate to the particular ones that are on the phone in order to select "'delete from device"?
I don't see a way to search for photos that are on device within photos.
I don't even see a way to tell if a picture is on device, other than to select it and see if "delete from device" option shows up.

If I could select all photos, I might be inclined to select "delete from device" accross the board (clear out all photos from my phone as long as they're backed up on g photos). But I don't see any way to select all photos.
pc photos doesn't seem much help. It doesn't even have any option to delete from device (and I'm not sure what it would mean if it did)

So you've got to go into google photos and identify which photos came from the phone, and

 

[electricpete]

I think I found a good tool to help reach my goal of cleaning my device (after verifying photos are backed up). The google photos app hamburger menu has an entry for "device folders" which includes only photos from my phone. It also groups them by their original directory (where they were on my phone). So if I focus on "delete from device" within this subset of photos (especially the camera directory), I don't have to wade through all my other photos (from pc).