How do I view the backed up photos from an older phone before deleting that phone from iCloud?

Sorry but it isn't possible to view the contents of an iCloud backup. The only thing you can do with an iCloud backup is use it for a complete restore to an iOS device. Doing the restore will erase the device and replace its contents with the contents of the backup.

However, if you suspect that the photos may have bee synced to iCloud Photo Library rather than backed up to iCloud, you can log in to https://www.icloud.com/ and view them in the Photos app there.