Log into your UA account at the UA site, go to your account profile, under travel preferences find the section on Display Preferences, change your setting from "a novice" to "an expert."

Among other things, you will be able to see all the fare buckets (booking codes) and their availability on every UA flight. This is covered in greater depth in many posts in the UA forum(s). One such link I found using the forum search tool is

http://www.flyertalk.com/forum/unite...-each-one.html

If you know nothing about booking codes or fare buckets, plan to spend a bit of time reading and learning. Well worth the investment (invaluable actually) if you want to know how UA allocates inventory and what fares are available.