Most people think they're backed up until the day they actually need it — and find out they weren't.
iPhone
Go to Settings → your name → iCloud → Photos, and make sure "Sync this iPhone" is switched on. For a full device backup, also enable iCloud Backup under Settings → your name → iCloud → iCloud Backup.
Android
Open Google Photos → your profile photo → Photos settings → Back up, and make sure it's turned on. For a full device backup, check Settings → System → Backup.
How to actually check it's working
Take a new photo, wait a few minutes on Wi-Fi, then check it appears on photos.google.com or icloud.com from another device.
Still not sure?
Bring your phone in and we'll check your backup is actually working properly — it only takes a few minutes.