how do I know my subscription has been cancelled

See the following from If you want to cancel a subscription from Apple - Apple Support

If you don’t see Cancel, the subscription is already canceled and won't renew.

and

If you cancel a paid subscription, you can keep using the subscription until the next billing date.

Also, a subscription that is already cancelled will typically have wording in fine print similar to "Expires on [date]" rather than "Renews on [date].”