how to identify on-device photos in photos app vs photos in icloud/ or google account

I'm sorry, but there is some real confusion, here. "on the device" could mean a number of things. Pictures can be in the Files app "on the device," but not in the Photos Library. (Pictures are not kept in the Photos app-- they are viewed using the Photos app.) Could you be referring to pictures seen in the Files app?

If you use Optimize Storage in Photos, then there may be a sense in which "photos are in the cloud" rather than "on the device." An Optimized Library may keep the originals at iCloud while smaller versions are "on the device." Could this be what you mean?

If you use iCloud Photos, then every picture you see on the iPhone you can also see at iCloud.com. If you have somehow acquired pictures that cannot be in iCloud Photos, then they will not be seen in the device's Photos Library.

If you don't use iCloud Photos, then you can certainly have pictures "on the device" that are not at iCloud.com.

If you use Optimize Storage in Photos, and if you turn off iCloud Photos, then you will be warned that any pictures on your phone that are "optimized" will be removed, though they will remain at iCloud.com. You are also given the option to download those originals to your device -- but that will take lots of storage.

See this:

Understanding iCloud Photos - Apple Community

iCloud Photo is a synchronization service.

If you are using iCloud Photos, and you delete a photo from your iPhone, you will also be deleting it from iCloud and from every other device (iPad, Mac, etc.) synchronized with iCloud Photos for the same Apple Account.

When you are using iCloud Photos, you can turn on "Optimize iPhone Storage". This gives the phone permission to substitute lower-quality, space-saving local copies of photos for full-size local copies of photos (and vice versa).

I'm not aware of anything on the phone that would tell you whether a particular local copy is a space-saving one or a full-size one.

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You seem to be wanting help with a Google app, is that right? This "save to device" doesn't seem to be from any Apple app?

So what is confusing to markwmsn, to Servant of Cats, and to me, all of us pretty experienced users, is the statement "i have some photos that are in the cloud that aren't on the device." We don't see how that can be true.

We are also unsure of the "save to device" command, and what it means. Mostly, if a web site, for instance, has you save a picture, it goes into the Photos Library (again, not into the app.) Then Photos takes over, doing what we've discussed, with the local Library and the Library at iCloud.com.

As far as pictures that you've backed up to your computer, there's no way that Photos would keep track of that backup. The computer backup does, though. If I wanted to collect the pictures that hadn't been backed up, I'd use the date and time of the backup. I would sort pictures on the iPad in order of date added, and select the pictures before or after the backup date.

If iCloud Photos is enabled on your device and fully synced, there are not supposed to be any photos in the iCloud Photos library that are not in the Photos library on your device. Any that were previously deleted from the device should be deleted from the iCloud library should be deleted from any synced device(s) as soon as feasible. Any that were recently added to the iCloud library from another source should be synced to device as soon as feasible. Any exception to that process would be considered an issue that needs to be explained and corrected, but there are few user-level tools to view and manage those exceptions.