While wine-tasting is the name of the skill, it is just as important to be able to evaluate a wine from the way it looks in your glass. For this, your glass should be about a third full. It is possible that through experience with this, you can even learn to identify certain varieties of grapes from their appearance (probably combined with other factors, like scent). A saturated, purple-black color wine is likely to be Syrah, while a lighter shade could suggest Pinot Noir. This knowledge can only really be gained through experience and guidance from another experienced wine-taster, which is why wine-tasting courses are highly recommended to fully appreciate what you see, smell and taste.
By holding the glass in your hand and gently rolling the wine around the bowl, you can gain a full appreciation of the depth of color in the wine. The center of the wine will be darkest while the edges will give you more of an idea of the range.
By holding the glass up to the light, you can judge how clear it is. Murkiness can be a sign of less care being taken in the wine-making process, although some wines are unfiltered and sediment can be seen in wines that have been shaken up slightly due to transport or clumsiness.
If you tilt the glass so that the wine lies in a thin layer toward the rim, you can judge the age and weight of the wine. Pale and watery color indicates that the wine is quite weak, while a rusty color can indicate that the wine has oxidized and is thus past its optimum through aging or mishandling.
Finally, the stereotypical wine-tasting trick: swirl the wine in the glass. If this produces little ‘tears’ or ‘legs’ on the glass, it is a sign of a wine with higher alcohol or glycerin (sugar) content. These wines tend to have more weight and be ‘riper’ than other wines.