See below for step-by-step instructions, followed by a video tutorial.
One last, very important step before publishing your episode is to use the Preview function in Megaphone to do a quick spot-check and make sure you've got everything set up correctly. This is specifically important if ads are targeted at your episode to ensure you have the correct ad locations setup within the episode. See this article for info on how to define ad locations.
1) Navigate to the "Edit Episode" page.
Start by going to the Podcast tab, click on the show you'd like to Preview an episode for, click on the episode that you'd like to Preview (for already published episodes, you can access the Preview page by clicking "Edit Episode").
2) Once on the Preview page, use the play icon to listen to the episode as it will be published.
You can navigate to different parts of the episode by clicking in the gray waveform.
3) If you have ads targeted to this Episode they will be listed below the waveform.
You can also see a color-coded visual representation (the purple and red spots in the picture above) where the ads have been placed in your episode.