This question already came up fairly many times, and I've answer to every single thread whenever I see it.
Very simply, you can check out your reservation on Apollo CRSs, by which can't be used unless you're an agent.
On the internet, there is no way to pull out a reservation by a record locator.
However, if you are a member of Mileage Plus and your reservation is holding your Mileage Plus number, you can check the reservation on www.united.com.
United uses Apollo.
Apollo shares the same host with Galileo, but their reservations can't be displayed on www.viewtrip.com, which can display Galileo reservations.
It would better to post this thread as a sticky, IMHO.