Originally Posted by star_world

Aircraft number is on the flight status page, so you can see it from two days out. On co.com (and mobile.united.com now) you can see specific cabin configurations and amenities for that aircraft, including E+, DirecTV / AVOD, etc. It's extremely useful and accurate in my experience using it over the last few years.

Aircraft number is slowly being added for UA metal operated flights, from what I can see, but not the corresponding amenities yet. So in the future there should be no need for the non-intuitive solution of going to a cargo website.