Noflex carryon enforcement?

On a noflex fare the carryon (as opposed to one personal item) is supposed to be subject to an additional fare. But how do they enforce this? Last flight I paid for the carryon ahead of time, but noticed that nothing on my boarding pass indicated that I was allowed a carryon and no one seemed to be checking at the gate as I board.

So when/how would it turn out if I just tried boarding with an unpaid carryon?