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=1647&fli ghtDate=11%2F30%2F2015&origin=EWR&destination=SFO

https://mobile.united.com/FlightStat...estination=SFO


Just change the flight date to match the day of your scheduled flight and then change the origin / destination as 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.

You may also check the specific fleet variant of each flight at the United Cargo site under "Schedules & tracking". This data tends to be reliable starting 7 days in advance, but is always subject to change.

United Airlines Equipment (Aircraft) Codes (via JOSECONLSCREW28):

sCO
77Y - B777-200ER (Tail #s 001 - 022)
76P - B767-400ER (Tail #s 051 - 066)
78V - B787-8 (Tail #s 901 - 910, 912 - 913)
78Z - B787-9 (Tail #s 950 - 959)
75B - B757-200 AVOD (Tail #s 101 - 141)
75C - B757-300 DirecTV (Tail #s 851 - 859)
75A - B757-300 Streaming (Tail #s 860 - 871)
73V - B737-900ER (Tail #s 413 - 478)
73I - B737-900 (Tail #s 401 - 412)
73F - B737-800 DirecTV, SFP(Tail #s 501 - 518)
73T - B737-800 DirecTV, SFP (Tail #s 519 - 531)
73J - B737-800 DirecTV (Tail #s 202 - 239, 241 - 245, 247, 248, 250-292, 299)
73M - B737-800 Guam (Tail #s 201, 240, 246, 249, 293-298)
73A - B737-700 DirecTV (Tail #s 701-719, 721-724, 726-733, 750-754)
73P - B737-700 Guam, Overhead IFE (Tail #s 720, 734 - 736)

sUA
19S - A319 (New Slimline Seats)
20S - A320 (New Configuration Slimline Seats)
73B - B737-900ER (Tail #s 801- 810)
37K - B737-900ER (Tail #s 811 - 849, 880 - 894) (Slimline Seats)
47C - B747-400
57Q - B757-200 2-cabin (54xx Tail #s) - exiting fleet
57U - B757-200 2-cabin (56xx Tail #s) (ETOPS) - exiting fleet
67I - B767-300 3-cabin (Tail #s 6441 - 6463) (All aircraft to become 76E) (will become 76C if in 2-cabin configuration, will become 76N if in 3-cabin configuration with winglets)
75J - B757-200 ps (59xx & 53xx Tail #s) (will become 75K)
76E - B767-300 2-cabin (Tail #s 6664 - 6677) (will become 76C)
77D - B777-200 3-cabin (24xx Tail #s) - to get new Domestic config
77H - B777-200 2-cabin (Tail #s 2510 - 2515, 2368, 2372, 2379) (Hawaiian)
77G - B777A 2-cabin (see http://aviationweek.com/commercial-a...-777-retrofits) 28J/336Y with E- going 3x4x3 across
77J - B777-200 3-cabin (Tail #s 2704, 2706, 2786, 2791, 2792, 2793, 2795, 2796, 2799)
77Q - B777-200 3-cabin (20xx & 28xx Tail #s)

For reference: United Airlines Fleet Updates [2014 Thread]