In order to categorise an olive oil, taste is as important as the chemical analysis. Olive oil is the only food required to undergo a trained and approved panel test to prove its category statement.
To be able to label an olive oil as Extra Virgin, there must be no defects in its taste and must at least just perceptibly fruity.


Flavor defects in olive oil are associated with problems with the olive fruit (olive fly, frozen conditions), improper handling of olives during harvest (dirt, wet fruit, prolonged storage prior to milling), certain milling conditions (unsanitary equipment, excessive heat), and improper or prolonged storage after milling (oxidation).


The first step in learning how to taste olive oil is to understand how our senses work. Perception of flavor relies on both our senses of taste and smell. The ability to taste is quite limited; receptors on our tongue can only discern sweet, salty, sour, bitter, and umami. All other information that we think of as flavor is actually perceived by smelling food through the back of our nostrils (retronasally) while it is in our mouths.


And then, we undergo this process:


1) Pour two tablespoons into the blue olive oil tasting glass, or glass for white wine or a glass for cognac.
2) Cover the glass with your hand to help the olive oil warm up while simultaneously you cover the top of the glass. This will help release the aromas and flavours of the olive oil.
3) Swirl the glass for a couple of minutes.
4) Release the hand covering the glass. Instantly sniff a couple of times the olive oil and then take a deep breath to sense the fruity attribute.
5) Sip a small amount of the olive oil. Close your mouth well and draw in air through your teeth, delicately first and more intense after, while breathing out through your nose. Here you must sense the bitter attribute.
6) Now you can either expel the olive oil or continue by swalling the olive oil to sense other things about this olive oil.
7) Right after you swallow the olive oil, the phenols of olive oil with start a reaction which you will feel as a tingle in your throat and might be as strong to make you cough 1-2 times. This is the pungent atrribute which is absolutely normal to happen and it is a proof of olive oil quality.

Finally, here are some common attributes of olive oil:

Positive:


Astringent: dry and puckering mouthfeel created by tannins mostly appearing in bitter and robust olive oils
Buttery: creamy, smooth sensation on palate
Floral: perfume / aroma of flowers
Forest: fresh aroma reminiscent of forest floor, NOT dirty
Fresh: good aroma, fruity, not oxidixed
Fruity: refers to the aroma of fresh olive fruit, which is perceived through the nostrils and retronasally when the oil is in one’s mouth.
Grass: the aroma of fresh-cut (mowed) grass
Green: aroma/flavor of unripe olives
Harmonious: balance among the olive oil’s characteristics with none overpowering the other
Herbaceous: unripe olive fruit reminiscent of fresh green herbs

Negative:


Acetone: aroma of nail polish remover, associated with winey defect
Blue Cheese: aroma associated with muddy sediment defect
Burnt/Heated: caused by processing at too high a temperature
Cucumber: off flavor from prolonged storage, particularly in tin
Dirty: oils which have absorbed unpleasant odors and flavors of dirty waste water during milling
Flat / Bland: oils which have no positive or negative aroma or flavor characteristic of olive oil; may indicate presence of refined olive oil
Frozen / Wet Wood: sweet, dry, and untypical aroma/flavor derived from olives which have been exposed to freezing temperatures
Fusty: anaerobic fermentation that occurs when olives are stored in piles too long before milling
Grubby: flavor imparted to oil by olive fly damage to olives
Haywood: flavor of dried olives
Muddy Sediment: barnyard-like aroma caused by olives' prolonged contact with dirt before or after milling
Musty: moldy, humid flavor created by wet olives that have been stored too long before pressing
Metallic: oils that have had prolonged contact with reactive metal surfaces either during processing or storage
Rancid: the flavor of oxidation that occurs as olive oil ages, often described as “stale nuts”
Rough: nasty, thick, greasy mouth feel
Unbalanced: olive oils with overwhelming flavors of bitterness and pungency
Winey /vinegary: sour flavor caused by aerobic fermentation of olives during processing

Sources:
http://www.internationaloliveoil.org/
http://www.wikipedia.org
http://www.oliveoilsource.com