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The Fleet Status site is:

https://sites.google.com/site/unitedfleetsite/

At the site, you can find information about the interior features, livery, etc., for every plane in United's fleet.

To determine which plane your flight will be on go to mobile.united.com and then click on flight status. Put in the flight number for your flight and just have it use today's date (occasionally it'll say flight not operated on this date in which case just choose one of the other 3 dates it lists). In the address bar you'll see something like this:

mobile.united.com/FlightStatus/FlightDetails?carrierCode=UA&flightNumber=1205&fli ghtDate=9%2F22%2F2013&origin=EWR&destination=SFO

Just change the flight date to match the day of your scheduled flight and then change the origin / destination if necessary. Once you do that look in the details tab under aircraft # and if they have assigned a plane it'll say something like #3506 and the type of plane. Once you know that go to the United Fleet website listed above and cross-reference the four digit number against the "aircraft" column under the appropriate aircraft type.

Keep in mind that things change as they get closer and sometimes either they won't have assigned an aircraft yet or it'll be assigned #0 if it's a few days away and they are anticipating aircraft movement (e.g. putting a new/converted p.s. plane entering into service).