Devices
Check your phone photo backup before a cleanup
Verify a sample from another place before removing files from your phone.
Pick a sample you can recognize
Choose a few photos from different dates, a long video, and a picture you recently edited. Include anything you would especially want to keep. Record where the original photos currently live.
Open the destination separately
Use a different device, a web view, or a file browser to open the destination you selected. Load the samples and play the video to verify that the destination holds usable files. Check the resolution or quality where it matters to you.
Review sync and deletion behavior
Read the instructions for your specific photo app before removing a file. For synced libraries, deletion can also affect other devices and cloud copies. Keep an independent copy of anything important before making large changes.
Try a small batch
Once you know how the service works, clean up a small batch of files you can recover. Check the backup again and verify the storage change on the phone before you continue.