You can send an exact preview of the reminder directly to your email inbox before it is sent to your customer. This can be done before or after reminders have been enabled.
To preview before enabling reminders, search for and select any customer invoice. Then click 'Prepare instant reminder':
Now click the Preview & Send tab:
You will see an option to send a preview to your email address - click this button, you will then receive this reminder email to your inbox which will look exactly how your customer will receive it:
Another way to send a preview email to yourself is once reminders have been enabled from your Settings page. Simply navigate to your Outbox, click Preview next to any scheduled reminder:
Then click the Send Preview to: button. The email will arrive in your inbox shortly.