Email templates can be tested at any time. When adding or editing the email template you'll see a blue button on the bottom called Save & Send Test Email.
Click on that button and Expiration Reminder will save the template and prompt for an email where to send the test email or a mobile phone number where to send the test message.
Then indicate what expiration to use as testing. This is needed so the variables in the template can be replaced.
Then click Send to receive the test email.