Go into your itinerary on UA.com and select "enter email and request receipt" or something like that. You should receive an email within a couple of minutes. At the top (not in the flight segment area), you will see your seat assignments listed.

For me, this has proven the most accurate and reliable method to see what seat you are actually assigned. The "web displays" are notoriously glitchy and do not update as they should.

Good luck.