What's the best way to test changes to the email template before it is sent to customers?
The email template is live on Zendesk. Any changes that you make are immediately reflected in email notifications. It is not possible to test the email template prior to saving it. However, if your pricing plan allows it, you can make and test changes in a sandbox environment.
Alternatively, immediately after changing the email template, create a test ticket with a personal email to see the new changes and tweak the template as needed.