How can I tell if my phone is backed up?

To see if it is backed up to iCloud go to Settings, tap on your name, then iCloud, then iCloud Backup and it will tell you the date and time of the last backup.

To see if it is backed up to your computer, go to the initial step of backing up, and the backup status will be displayed on the iTunes or Finder Summary screen (depending on your computer type and operating system)→How to back up your iPhone, iPad, and iPod touch - Apple Support.