There are three elements to consider in evaluating a wine’s quality: its appearance, its aroma, and its taste.
Color and clarity
To evaluate a wine’s appearance, hold a half-full glass against a white background, such as a tablecloth, and observe the wine's color and clarity. It should be brilliant, rather than murky, and its color should be appropriate for its type and age .
Young white wines range in hue from a pale straw-yellow to a rich amber. The color depends on the grape variety, the ripeness of the grapes at harvest, the way the wine was fermented and aged (white wines fermented and/or aged in barrel will have a more golden hue than those fermented and aged entirely in stainless steel tanks), and how much oxygen the wine was exposed to during vinification and bottling. As they age, white wines darken, assuming a deeper golden color. Browning suggests either an extremely old white wine or, if found in a younger white, a wine that has been prematurely oxidized and should probably not be consumed.
Red wines, conversely, grow paler as they age. Young reds range in color from a translucent cherry for lighter wines such as Beaujolais or Pinot Noir to a deep ruby, sometimes with purplish tints, for a Zinfandel or Syrah. Older red may display a brickish hue around the edges. This should not be present in a younger red wine.
The most important and revealing aspect of a wine's personality and quality is its smell. Indeed, most of what we take to a wine’s taste is actually its aroma. Think of how the taste of food changes when you have a bad cold and can't smell.
When you swirl wine in a glass and sniff it, the volatile essences of the wine are carried by thousands of nerve endings in your nasal cavity to the olfactory bulb in your brain. The same thing happens, via the retronatal passage in the back of the mouth, when you sip and swallow wine. In effect, flavors are odors in your mouth. Swirling volatilizes the wine's aromas and sniffing draws them into the olfactory bulb, which "interprets" them -- i.e., compares them to other familiar smells.
This is a complex process, because a wine consists of over 300 different chemical compounds, many of which are identical or similar to those found in fruits, vegetables, spices, herbs, and other substances. That's why wine aficionados describe a wine’s aromas in terms of various fruits, vegetables, herbs and spices (e.g., apple, melon, citrus, cherry, berry, honey, peach, mint, bell pepper, grass, green olive, clove, licorice, cedar, etc.). They're not being fanciful; there actually are chemical correlations underlying the comparisons, which explains the rich metaphorical language used to describe a wine's sensory characteristics.
The primary grape smells of a wine, distinct by variety, make up its aroma, while secondary characteristics, caused by factors such as fermentation and oak and bottle aging, blend with its fruit smells to form the wine's bouquet.
To fully appreciate a wine's aroma, gently swirl it in your glass. The aroma should be clean and fresh, with the characteristic scents associated with the variety (e.g., apple with chardonnay, melon and citrus with sauvignon blanc, cherry and blackcurrant with cabernet sauvignon, blackberry and black pepper with zinfandel, etc.), perhaps accented by the toasty, vanilla-y, or spicy scents imparted by the barrels it was aged in. If the wine is older, it may have a less fresh and fruity aroma, but one with more complexity.
Although taste, as described above, is essentially a function of smell, tasting reveals aspects of a wine's personality that smelling cannot.
Humans can perceive combinations of only four tastes: sweet, sour, bitter, and salty. These sensations are localized in taste buds on different parts of the tongue: sweetness on the tip, with acidity and bitterness on the sides and to the rear.
You may notice the tartness of a young dry white wine, for instance, or the astringency of a full-bodied young red. Some varieties, like riesling, chenin blanc, and gewürztraminer, are especially fruity, if not literally sweet, while a strapping young cabernet sauvignon may taste very dry and even bitter.
The “body’ of a wine – the sense of its weight on your palate – is a function primarily of its alcohol content (the higher the alcohol, the weightier the wine). A wine too high in alcohol may taste hot or harsh, while a wine low in alcohol may seem thin or watery.
In red wines, the tannin acids contained in the grape skins, which are absorbed into the wine through fermentation (whites do not ferment on their skins and thus have little tannin) can promote a drying, puckery sensation, like sucking on a walnut skin, while a wine low in acidity may have an overly soft, even flabby impression.
Whatever tastes a wine imparts, the key to its quality is balance, a harmony of all its elements: fruit, alcohol, acidity, wood (if any). By tasting a wine – especially by sloshing it about in your mouth before swallowing - you can gain a quick impression of its most salient elements (is it fruity, tart, soft, bitter?) and to what degree those elements are in harmony. Combined with assessing its appearance and aroma, this will complete your evaluation of the wine’s quality and, most important, whether you find it pleasing or not.