This assume that Back up my data is set to ON under Settings > Backup & reset with a valid Google account (same as the rest of the phone). And that it’s using Android Marshmallow (6.0) and up.
Ok, it’s on but where do I see what it have backed up?
Open the Drive app, swipe from the left , or tap the 3 lines on the upper left hand corner. Then Backups, and then you select your device, as it will show all devices being backed up using that google account.
If you just turned on backup it might take a while for the backup to be done. Edit: backup will happen only when it’s on power and connected to wifi and been idle (not used) for at least an hour. Connect to charger and wifi and wait an hour or 3.
My M8 shown had backup set to OFF and was enabled just shortly before those screenshots.