how do i know that my phone is fully backed up?

If you backed up your iPhone with the use of a computer, then you will be notified that the backup has been completed. There will be a time stamp displayed.

If you use iCloud for backups, then you can verify the backup by opening Settings ➡️ Apple ID (Your Name at Top) ➡️ iCloud ➡️ iCloud Backup: Here, you will see the time when the last successful backup was created.

For more informaiton click here 👉 To learn how to back up your iPhone.

Axel F.

You will never know it. So, back it up every day. That's what I do.

Every time you add or delete something in your device, say add an app, delete an App, take a photo, Add/Delete a Note, Reminder or record audio, or delete a video.

Next time you take a backup the backup process will exactly the same add an app, delete an App, take a photo, Add/Delete a Note, Reminder or record audio, or delete a video.

So your backup will be a replica of your device data.