We last reported about Microphone / Webcam setup options existing in Skype Preview settings on Windows 10 Mobile. But these settings were not working (at least on my Lumia). Now, with the latest update you can setup and test Microphone and Webcam before using it for calls.
As you can see in the screenshot you now have a box for testing video after setting up default camera. You can set FFC or Rear camera as the default for video. Toggling tthe Test video on enables the preview of whichever camera you choose for. Test audio option also works fine after the latest update.
The latest version for Skype Preview on Mobile is 11.9.261.0. These features have been available on Windows 10 app for sometime. The update is available for Fast Ring as of now.