To be able to preview content of web content related FEED, one have to:
1. go to Web Content (in Control Panel or Site Content)
2. click Feeds tab
3. choose Edit from Feed action button
4. scroll all the way to the bottom of page
5. click Preview button
Suggested improvement would introduce Preview option in Feed action button. That would shorten preview case for 3 steps what makes users and testers happier.