How do I know my iPad is backed up?

If using iCloud to backup your iPad, you can easily verify the existence of the backup - and the date and time of the last backup - from iPad settings:

Settings > [Your Name / AppleID] > iCloud > iCloud Backup >

You can find all your device backups listed here - along with more information:

Settings > [Your Name / AppleID] > iCloud > Manage Storage > Backups

To see specific information about each device backup - and control which Apps and data are included within the backup - simply tap the corresponding backup entry. For example:

You should note that you can only configure element backup from the device to which the device backup relates. You cannot, for example, configure your iPhone backup from your iPad.

How do you back it up? iCloud or your computer? Whichever method you use, you check there to see the latest backup.