In preparing to judge beer based on style, it is important to know the styles. The BJCP styles are available on their website (http://www.bjcp.org/stylecenter.php). There are downloadable formats as well as browseable online formats. One that I find really handy for judging competitions is a printable PDF with a handy breakdown of expectations (http://www.bjcp.org/docs/StylePresentation.pdf). You can just pull up a single sheet for a style sub-category and have a quick reference to what levels are expected for a style for each of the judging criteria.
In some ways, Aroma is the most important aspect in judging a beer, despite flavor being worth more on a BJCP score sheet. It is the first part of the score sheet for a reason. When it is first poured, you should always check aroma. There are some transient aromas which may only be there on the initial pour but dissipate quickly. Some lagers will have an initial sulfury or skunky component which disappears. That initial aroma is important to note especially if it is expected or horribly out of style. Determining a score for aroma is based on whether it is supposed to be there or not and the level. The clue words to consider are malt, hops, esters, and other aromatics. As a general procedure, always evaluate aroma first. Jot down all of your initial impressions. In the case of a tasting exam, keep this under a minute. Get all the initial aromatics described, and then move on to appearance. After appearance, make a quick revisit to aroma and jot down any changes and assign a score based on how well it conforms to style. Now, what are we generally looking for...
Malt aromas generally range from light sweet corn to bready to even burnt. Generally the toasting level of the malt or type of malt will define the aroma and how much used will define the level. Pilsner malt has almost no toast level and is high in the precursor chemicals to Dimethyl Sulfide (DMS). Generally this is boiled off as long as the boil is long enough, vigorous enough, and uncovered. However, there will still often be enough left behind to smell like sweet corn. German lagers and some american lagers have a bit of this as part of their target profile. Higher levels or those caused by a bacterial infection may smell like cooked cabbage. This is almost universally a flaw. Higher kilned malts will generally have aromas more in line with their darkness. They may range from bready and biscuity to toffee to chocolate to coffee or even charcoal. How appropriate they are depends on the style. For example, an American Barleywine should have a high level of sweet, caramelly, bready malt aromas. By way of comparison, a Light American Lager should have low grainy, sweet, corn-like aromas. Some dark malts can even come off as somewhat fruity. For example, some stouts can have dark cherry notes to them (although this can also be a yeast byproduct).
Hop aromas are typically most notable in certain styles. IPAs are almost defined by their hop aromas. In my opinion, hops in beer are the closest thing to the wine term 'terroir'. In the wine world, the grapes pick up characteristics of the soil in which they're grown that comes through in the finished product. In much the same way, hops grown in certain regions tend to have certain characteristics. Pacific Northwest hops tend to have a lot of piney, resiny, and citrusy notes to them. German hops tend to be more spicy and somewhat floral. English hops tend to be earthy and sometimes floral. Hop aromatics can also be quite delicate and fleeting which is why you should always evaluate aroma first. On the negative side, some hop aromas can be negative (or perceived that way by some). Sunlight can react negatively with hop oil compounds to produce a skunky aroma. On the other side of the spectrum, some hops have aroma profiles which are expected but come off as negative to some people. There is a variety of hop called Simcoe which some people describe as 'catty' or even worse, 'cat piss'. Summit hops can also come off as 'oniony' when used for aroma. Good, bad, or indifferent, the important thing is to note what you're sensing.
Esters are generally a byproduct of the yeast. Certain yeast strains produce certain aromas. Most ale strains produce fruity esters. Kolsch strains produce apple or pear-like aromas. Belgian strains may produce banana, clove, or even cherry-like aromas. Saison strains may have lemony and spicy aromas. Generally lager styles should not produce any fruity esters, but can produce other aromatics like burnt matches. I even remember a California lager produced by a brewer in a club I used to belong to that smelled like spent gunpowder initially. It dissipated quickly after opening, but it was like a day at the gun range upon first pouring.
Other aromatics is a catch-all for anything else. There are check boxes on the left side of a score sheet for most of these with descriptions. On a tasting exam, only the key word will be there with no description. You should always check off the boxes when you sense the descriptor, regardless if it's to style or not. Acetaldehyde is a green apple-like aroma which is rarely to style. To get an idea of what it smells like, cut a fresh granny smith apple. Or just open a bottle of Budweiser. Their yeast produces an unusual amount of acetaldehyde which has become a part of its expected character. Alcohol also has an aroma. It can be warm, or even hot. Barleywines and stronger beers would be expected to have a pleasing alcohol warm aroma to them. However, sometimes alcohol aromas can be hot or solvent-like if produced by too warm of a fermentation. Diacetyl is an aromatic that can smell like butter or butterscotch. Sometimes it can be hard to tell the difference between diacetyl and the aroma of kettle caramelization. In those instances, one can rely on mouthfeel to help make the determination since diacetyl also has a slick oily feel. DMS we've already touched upon, but it should also be considered an 'other aromatic' in that it's often a flaw. Grassiness is also a potential flaw. While some hops have a grassy aroma, the aroma of a fresh mown lawn is not always appropriate. Light-struck beer is that skunky character mentioned under hops. Metallic is another potential aromatic. Think of copper, iron, or even blood. This is almost universally a flaw. Musty would be akin to an old basement or storage shed. Stale and moldy. Oxidation is an aromatic which will come off like wet cardboard or sherry. In general it's a bad thing, although in Old Ales it may be expected. Phenolic can come off in good ways for some styles (clove or pepper), but can also be smoky, plastic-like, or even like band-aids. Solvent is just what it sounds like. Nail polish remover or paint thinner. It can come from chemicals in plastic, infections, or even just fermenting too warm. Sour or Acidic can come off pleasantly tart or harsh and vinegar-like. Sulfury smells can be produced by yeast or infections and may range from rotten eggs to burnt matches (or spent gunpowder). Vegetal aromatics like cabbage, onion, celery, asparagus, etc. may come from a variety of sources. Finally, yeast itself has an aroma which should be noted if found (think of the smell of rising bread).
In summary, the important thing to remember about aroma when evaluating beer is to evaluate the beer in front of you. Jot down everything you smell. Note how prominent it is. Only mention things you don't smell if they are missing or relevant (for example, if you don't get any malt aroma in a barleywine). Aroma is not the place to start critiquing process. Leave that for your Overall Impression feedback. Score the beer out of 12 points based on how closely it hits the style target. One thing to remember is that typically proctors are fairly lenient by design, so while you shouldn't give every beer a 12/12, don't give a beer 3/12 unless it's seriously flawed or infected. Key points: remember to get initial aromatics right away, note them, go through the appearance section, and then come back to aromatics and assign your score. In my next article, I'll go through appearance.