Congratulations! You've flown the 75,000 miles on United and you're the proud owner of two regional premier upgrades (RPU's). Or you're very charming and have convinced someone to apply one of these upgrades for you. Either way, for those new to the elite status ranks, these upgrade instruments allow you to get priority in the upgrade queue or in some cases, allow upgrades that normally wouldn't be eligible for complimentary bumps up. The best examples of these flights not eligible for complimentary upgrades would be the new PS flights from San Francisco and Los Angeles to JFK in New York. Flights from the East Coast flights to Hawaii are similar in not allowing complimentary upgrades.
Seeing as these flights tend to be long and also have internationally configured lie-flat business class seats, they make the perfect opportunity to use your hard-earned RPU's. One of the best tools in seeing the upgrade availability is through the use of United's "expert mode". This option allows you to see the number of fare classes that are available in any given flight. This is extremely helpful because United designates a certain amount of seats for flights under "R" and "RN" classes, which are the classes you'll need to immediately apply your RPU.
Log in to your United account and then go to this page. Once you click through, go to the bottom of the page and click the button that says "I acknowledge these considerations and choose to enable Expert Mode". Once you do that, you can search for flights on the United search engine and can see exactly how many seats are in the various fare classes. Click on the "fare classes" link and you'll see a detailed rundown. If you see any "R" or "RN" classes with numbers next to them, you'll know that upgrades are good to go.
If you want to know more about United upgrades in general, click here: