The first part is tasting: are you able to discern the aromas, flavours and textures of what is in the glass? If you are in some way sensorially impaired, it may be that you can’t pick up some of the details. I knew a well-known UK-based Master of Wine who admitted to not being able to discern cork taint. He was able to say that the wine was stripped of flavour and dull, but he could not identify the actual smell and taste of the compound (TCA) which had destroyed the normal aromatic profile of the wine.
Assuming that there is nothing wrong with your smell and taste (the former being vastly more important than the latter), you still need to be able to interpret what it is your senses have identified.
At an elementary level this means correlating the colour of the wine with its overall condition (older red wines go brick-red at the rim – but does the wine taste youthful, mature or over-the-hill?) recognising the sweetness level as appropriate or not, establishing whether the mouthfeel is right or jarring.
At a more complex level it involves appreciating the wine for what it is (the label says Chardonnay, but does it taste like Chardonnay?) and then deciding whether or not you like it for what it purports to offer.
The final stage of this kind of appreciation requires that you understand the artfulness with which it was made – no different from looking at a painting, understanding the artist’s intention and then deciding whether or not he has succeeded.
Much is made of those blind tasting superstars who can pick up a glass of wine and instantly tell you where it comes from, when the fruit was harvested, and possibly even who made it. It can be done – but not all the time.