Anyone willing can be a judge, provided they are not blind (though some pilots would say this is not unusual in a judge!). If you are an interested pilot, or already a competitor, then also being a judge will give you much better insight to what you are trying to achieve while flying. But if you are not a pilot, don't be discouraged, many top class judges are non-pilots just like you!
The great benefit of being a judge is that all the flying takes place solely for your benefit and you are always in the best position to admire the flying, regardless of the sun position and wind direction - you are never stuck end-on to the box looking into the sun trying to gauge how a flight is going.
Judging experience and training can be obtained at Chapter flying training days. Where there is demand, we run Judge Training seminars. The first is an "Introduction to Sport Aerobatics". It is a basic intro to competition "Aresti" style aerobatics aimed at anyone interested in becoming involved with the sport. We provide some background on how the sport is run and what we are trying to achieve, how to read a sequence card, and introduce the scoring system basics. You will get to watch some video footage of aerobatic flying and see the resulting score.
The second seminar - "Basic Judging" - gets into the nuts and bolts of judging the basic figures that you will see in every flight. After this lesson, and with some practical experience at training days, you will know enough to judge up to Sportsman Category.
The third school - "Advanced Judging" - focuses on composite figures and those seen at the higher levels of competition, like rolling circles, tail slides and advanced flicks. We also get more into the rulebook regarding other penalties such as "lows" and "hard" and "perception" zeroes (what are they? - you'll have to come along and find out). The Fairplay scoring system is also covered. At the conclusion you will complete the AAC Judges Home Study Course and be ready to judge at a State Championship, a significant milestone after a few months of intermittent effort!
An excellent primer on aerobatic judging is freely available on the British Aerobatic Association website.