After you change browser engines, if you quit and then open a new browser window from the Globe on the desktop, you can tell what engine is active. If it's Opera, it will be a blue-themed Tableteer home page. If it's Mozilla, it will be a yellow-themed page with the Mozilla logo.

Is this what happens on yours? It should work this way if you do have the menu option. It's hard to tell which engine you're using, because both engines use the same identical browser app. Also, if you go to a page with RSS feeds, the Mozilla browser will have the orange 'RSS' button down at the very bottom, while Opera does not.

Make sure you quit the browser and relaunch after switching engines, otherwise it won't take effect.