Test it as a viewer
As soon as you open the test webinar you can choose "As attendee" to your right to watch the webinar yourself (on a second screen or in a second browser tab).
Refresh the page when you no longer see the option.
Please note: Make sure you work with some headphones or turn off the test viewer sound. This avoids sound from the test viewer being picked up by your microphone (which creates an echo effect).