Professional olive-oil tasters score an oil on three positive attributes — fruitiness, bitterness and pungency — and the absence of defects. You can do the same at home with nothing but a small glass and your attention. Here's the method panels actually use.
Pour a tablespoon or two into a small glass — tasting panels use cobalt-blue glasses specifically so colour can't bias the judgment, since colour says nothing about quality. Cup the glass in one hand, cover the top with the other, and warm it to roughly body temperature for a minute. Warmth releases the volatile aromas.
Uncover and inhale deeply: a fresh oil smells green — cut grass, tomato leaf, green almond, artichoke, ripe fruit. Then take a small sip and 'strip' it: with the oil on your tongue, draw air sharply across it through pursed lips (a slurp). Aerating the oil this way spreads it across the palate and lets you sense the back-of-throat attributes.
Fruitiness is the aroma and flavour of fresh, sound olives — green or ripe. Bitterness is tasted on the tongue, especially the back; a clean bitterness is a virtue, a sign of fresh polyphenols. Pungency is the peppery sting at the back of the throat that can make you cough — again a marker of healthy phenolic compounds, not a flaw. A great oil balances all three.
The same method exposes faults. Rancid (stale, waxy, old-nut); fusty or muddy (olives that fermented in the heap before milling); winey-vinegary; musty (mould). A single sip of a fresh, lively oil next to a tired supermarket bottle teaches the difference faster than any description.