How do I know if the Hide My Email option was used when setting up an account?

You can find any Hide My Email addresses that you may have already set-up from iPad settings:

Settings > [Your Name / AppleID] > iCloud > Hide My Email

Each entry will have its own set of details - from which each account can be managed as required.

Where accounts have been created to log-in to websites, you may also find a corresponding entry within your Saved Passwords (Keychain):

Settings > Passwords

More information:

Set up and use Hide My Email in iCloud+ on all your devices - Apple Support

Learn more about Hide My Email and iCloud+ --> Learn more about Hide My Email and iCloud+ - Apple Support

"Hide My Email is a service that's included with your iCloud+ subscription. With Hide My Email, you can generate unique, random email addresses that automatically forward to your personal inbox, so you can keep your email address private." "You can create as many email addresses as you need."

Refer to Set up and use Hide My Email in iCloud+ on all your devices - Apple Support

To turn this off, do the opposite of what it says for setting it up.

This largely depend upon the reason that the Hide My Email address was created…

If the address was created for a website - where you had to create an account with a Password - you will need to sign-in to the corresponding website where you created the account. Having signed-in to your account, you may be able to change the email address associated with that account.

If instead the Hide My Email address was created in response to registering an email to receive marketing or other mail messages - but was not prompted to create an account - you’ll not be able to update the email address. Instead, you will need to re-register with the service, using your real email address.