How can I tell what photos I don’t have on my device but are in icloud?

iCloud is a syncing service, as such, there cannot be anything on iCloud that is not also on your device. The Photos app on your iPhone will show all photos, and will naturally sync them all to iCloud if space permits.

You can have lower quality photos on your device than on iCloud if you have Optimize Storage enabled in Settings ➜ Photos.

Not sure what messages you are seeing. Turning off iCloud Photo Library will not delete anything from iCloud. It will just sever the connection between iCloud Photos and the iPhone Photos App.

This different from turning off iCloud entirely.

Normally, when turning off iCloud from Settings ➜ Account ➜ iCloud you should get a prompt asking you if you want to keep your content on the device or delete it. Answer that you want to keep it, to not delete anything from the device.

Once you turn it off, you can always go to the iCloud.com website, and delete anything from there without affecting any devices since the iCloud sync would have been turned off.

Where exactly on the iPad are you seeing the message that says you will lose 9000 items?

Can you post the complete message?