Starting about 3 days prior to the flight you'll be able to see the preliminary fleet number of the aircraft they think will be operating your flight from the flight status section of the app or United.com.

In the inflight amentities section you can see if there will be seatback entertainment as shown below, that will be an indicator of whether the aircraft at that moment assigned to your flight will be NEXT or not.

As others indicated it can change so no guarantees.