Navigate to the desired Email Template. Select the Send Test And Verify Merge Fields button:
When the pop up window shown below appears, consider what type of Record the Email Template will be sent from. In this Example, the Appointment Confirmation Email Template is sent from the Appointment record, so we'll choose Appointment. If I were testing the Paid in Full Confirmation Email Template, I'd chose the Sale record.
For this example, in the 'Related To' Record dropdown box choose Appointment, and use the Lookup field to select a specific Appointment.
Check the checkbox for 'Send email preview to' and enter your email address. Check your email inbox to see the results of the test email.