I commuted in and out of DEN and would sometimes take OO flights operated for United. I had no ability list ahead of time at that job, so I just went to the gate. They always helped me out once I explained that I could not list ahead of time. However, I never checked bags.
If you go to the gate note that the agents are usually a single person working each flight and they don't show up until a bit closer to departure time than a mainline flight.