You do it every time you drink a beer. You may not realize it but you are. You are judging.
Of course, a formal judging has specific parameters and requires a much more analytic method. Mouthfeel, volumes of Co2, yeast character, hop aroma, color in degrees SRM, etc.
If you think you might enjoy a formal education in analyzing your favorite beer and learning proper characteristics of dozens of classic styles, The Beer Judge Certification Program has all the resources you need to become a industry-accepted judge.
Consisting of a preliminary online test followed by a sit-down tasting, participants have the opportunity to achieve various levels of certification. Higher levels must not only score well on the exam but must also have a certain number of competitions judged and/or hosted.
Study hard if you plan to take the test... the exam is designed for serious beer enthusiast.