At our shop extra virgine olive oil tasting is very easy, we try to compare 3  to 4 different quality's with dipping bread in each olive oil. Usualy we start from milde, medium body and little bit more stronger. Taste is very personal and every body will choose different quality. It is not metter of quality actualy but metter of personal preference.

But if you want to taste olive oil in professional way under you have some descriptions.

The visual aspect is negligible in the evaluation of an oil, so it tends to make sensory analysis using glasses that cover the color blue.
Pour into the cup, a small quantity of oil, cover with one hand and warm it with the palm of the hand.
Remove the hand that covers the glass and bring it to the base of the nose while breathing three times in a discontinuous manner, not to accustom the taste buds. At this stage, try to memorize the smells arising from inhalation.
Put in the mouth a quantity of oil equal to 8-10 drops and warm it for a few minutes, so as to favor the evaporation of volatile components.
Inhale air from your mouth in a determined and strong, making sure that the oil will not go down his throat. This technique, called stripping, allows the oil to oxygenate and feel their flavors more intense.
Make swirl the oil in your mouth long enough to place him in contact with all the taste buds.
With heating, oxygenation and swirling you can perceive more accurately the strengths and weaknesses of the oil.
After these steps, the oil will be distributed throughout the oral cavity and especially on the tongue from tip to tail, and from the margins to the dorsal.
At this stage, pay attention to the type and the order in which sensory stimuli are presented: those that include tactile smoothness, texture and oily, to the purely gustatory, affecting the perception of different tastes: sweet , bitter, spicy.
this is done, eject the oil.   
Bitter: characteristic flavor of oil from green olives or olives turning color, which must not be over-emphasized.
Artichoke: flavor reminiscent of the taste of the softer part of the artichoke.
Dessert: attribute suitable for oils not very pronounced flavor, aroma of subtle, delicate.
Flowers: delicate flavor of white or yellow flowers, gives a delicate flavor oil.
Fruity: flavor which brings to mind the smell and taste of a sound, fresh fruit picked at optimum ripeness. On tasting, the fruit can cover a range that extends from a mild degree (softer) to a very intense (more pungent).
Almond: typical flavor of almonds, fresh or dried. It is appreciated as a retro feel and smell and is typically associated with sweet oils.
Mature: characteristic flavor of ripe olives, often present in oils from the bright yellow and round taste, almost sweet.
Green oil: generally suitable defining the Tuscan oils, fruity aroma from the large and bright green.
Pungent: stinging sensation characteristic of oils produced at the beginning of the year, primarily from olives that are still green. It is a pleasant and characteristic note of the oils from Tuscany, Puglia and Sicily: To be of value should not persist too long in the mouth.
Round: attribute suitable oil that has a full flavor and balanced
Spices or vegetables or apple, if the dominant oil respectively evokes these flavors and scents
Green grass: when the oil has a flavor reminiscent of the smell of freshly mown
Vegetable water: flavor acquired by following a prolonged contact with vegetable water
Heated or burnt: smell the oil that has been exposed to high temperatures during processing
Hay-wood: characteristic odor of oils extracted from olives that are now dry
Rough: attribute suitable for oil in the mouth after swallowing a feeling of dirt
Metallic: flavor reminiscent of metal and which arises as a result of a pervasive or prolonged contact with the olive processing machinery
Sludge: defect that occurs in the oils are not properly filtered and, remaining in contact with the established (or 'bottom sludge'), determines the formation of unpleasant odors
Mold-humidity: perceptible flavor oils extracted from olives harvested in the ground (sometimes contaminated by microorganisms in the soil, such as fungi and yeasts) and stored for a long time in damp
Rancid: is found in oils that have been exposed to air and heat, is accentuated with aging.
Heated: defect due to improper storage of the olives, piled, ferment sending this scent oil from them.
Svinato-vinegary: flavor reminiscent of that of the wine or vinegar, characteristic of oil that has undergone a degeneration fermentation and the subsequent formation of acetic acid, ethyl acetate and ethanol. Alternatively it may be due to the practice, still common enough, using vinegar to wash the tanks of harvest.
Earth: the specific scent of an oil obtained from olives harvested and washed ashore so inaccurate.
Grubby: annoying flaw that can be found in oils derived from olives infested with colonies of parasites.