Browse to the Meeting Info page in the Skype Meeting Broadcast portal.
Scroll to the bottom of the page to the "Media distribution " heading.
You should see the text "Software-Defined Network is enabled".
If you do not see this message, it's possible Hive for Skype is not properly activated on the Skype Meeting Broadcast instance. Check with your Skype or Office 365 Administrator. It's possible the Hive for Skype activation described here has not been completed.