How do I know if I cancelled an app during the three day trial period?

Cancel the same way as a paid subscription. See instructions in If you want to cancel a subscription from Apple - Apple Support

If you think that you have already cancelled, see the following from If you want to cancel a subscription from Apple - Apple Support

You might need to scroll down to find the Cancel Subscription button. If there is no Cancel button or you see an expiration message in red text, the subscription is already canceled.

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