There are five main criteria used when judging beer in competitions: aroma, appearance, flavor, mouthfeel, and overall impression. The total possible score for a beer is 50 points. Aroma is worth up to 12. Appearance is worth up to 3. Flavor is worth up to 20. Mouthfeel is worth up to 5. Finally Overall Impression is worth up to 10. You should rank these relative to the style guidelines available on the BJCP Styles. In other words, no matter how good the beer may be from a simple enjoyment perspective, the scores should reflect how close it adheres to the guidelines for that style.
For aroma, a beer in a hoppy style like IPA should have the aroma characteristics of an IPA, and moreso of the substyle of IPA in which it's entered. If an English IPA smells like resiny pine or grapefruit, it is not to style, no matter how pleasant it may be. There are four general components to aroma: malt, hops, esters, and other fermentation characteristics. Each style will have expected levels of each, and levels which are considered out of style or off. Malt may have aromatic characteristics described as bready, toasty, cookie-like (or biscuity in British terminology), molasses, coffee, chocolate, roasty, burnt, nutty, etc. Hops can have aromas described as flowery, grassy, earthy, herbal, piney, resiny, minty, citrusy, pineapple, etc. Yeast will generally have fruity characteristics in ales and may be described as apple, pear, plum, raisin, cherry, etc. Bacteria or other characteristics may be described as sour, vinegar, horse blanket, etc. All of these are evaluated on whether they belong. For example, a lambic or Flanders Red should have a sour component to it, but a Strong Scotch Ale should not. You would award the appropriate percentage of the 12 points based on how well it represents the style.
Appearance is a relatively smaller range of variables. You're basically looking for whether the color, clarity, and head retention and texture correlate with expected guidelines. A Light American Lager will look nothing like a Russian Imperial Stout. Also, a Witbier and a Standard English Bitter will have a different expected amount of carbonation and head retention. Since there are only 3 possible points, it's easier to assign a score. 3 is perfect for style, 2 is so-so, and 1 would be not appropriate for style.
Flavor as the largest percentage of total score is obviously the most important. Only aroma (which is also a part of flavor) is close to the weight of flavor. Again, malt, hops, yeast, and other characteristics make up this one. Malt brings many of the same flavors as it brought in aroma with some sweetness characteristics. Hops, in a similar vein, bring bitterness to the table. Infections and poor handling can also bring new components such as cardboard or vinegar. Additional comments should be made on the finish, balance, and any aftertastes. You should also mention any flavors that are missing or present only when appropriate for style.
Mouthfeel is also a relatively small portion of total score. You are basically looking for body, carbonation level, creaminess, astringency, or any other palate sensations such as slickness (which is a sign of diacetyl).
Overall Impression is the editorial piece. In other sections, you are only supposed to report sensations and their level, presence, or absence. Impressions is where you discuss intangibles and provide feedback on how the brewer might get closer to style. When judging a competition, you should always address what they did well or at least offer some positive feedback before offering constructive criticism on improvement. You should never be harsh or rude. In most competitions, the entrants pay a fee to enter to get helpful feedback, so you should try to provide that.
In my next series of articles, I'll try to flesh out each of these sections and offer helpful tips on giving the best feedback you can.