Corked, cooked or just closed.

The sommelier pours a splash, you taste and the table waits. The only question is whether the wine is sound. Knowing how to tell if wine is corked is the heart of it. A corked bottle smells musty, like damp cardboard or a wet cellar, with its fruit muted or missing.

The other faults are just as learnable. Most announce themselves on the nose before the wine reaches your mouth. Below are the ones you are likely to meet, the harmless quirks often mistaken for them and what to do when you find one.

How to tell if wine is corked

Cork taint comes mainly from a compound called 2,4,6-trichloroanisole, TCA for short. It forms when moulds act on chlorine-containing compounds, classically in cork bark. The nose is remarkably good at finding it. Sensitive tasters can detect one or two billionths of a gram in a litre. White and sparkling wines give it away at lower levels than reds.

At full strength it smells musty: damp cardboard, a wet dog, a mouldy cellar. The subtler effect is worse. TCA also dulls your sense of smell. Research on the nose's receptor cells found that it suppresses their response to other aromas, even below the level at which it smells musty. A faintly corked wine may not smell of anything obviously wrong. It tastes dull, short and closed. It is easy to blame the wine instead.

A few checks help:

• Smell the wine, not the cork. A cork smells of cork and wine whatever its condition; the glass is what matters.
• Trust the first sniff. Your nose adapts quickly, so a faint taint can seem to fade as you keep smelling.
• Compare with a second bottle. If there is another bottle of the same wine, it is the surest test there is.
• Do not rule out a screw cap. TCA can also come from barrels, beams and other wood in a winery, so a wine under screw cap or a synthetic stopper can be tainted too, though less often. For what each closure means, see screw cap versus cork.

Oxidised, cooked or simply tired

Oxidation. A wine that has taken in too much oxygen, through a failed cork or through age, smells of bruised apple or caramel and tastes flat and short. The colour drifts too: whites deepen to gold and then brown, while reds turn brick and brown at the rim. Some wines are made this way on purpose. Amontillado and Oloroso Sherry, tawny Port and Madeira all depend on controlled oxidation; in a fresh young white, the same smell is a fault.

An old wine whose fruit has faded and whose structure is drying out is past its best without being faulty. How drinking windows work explains why that moment comes sooner for some bottles than for others.

Heat damage. In Hong Kong this is the fault to watch most closely. A bottle left in a hot car, a shop window or a flat without air conditioning tastes cooked: stewed, jammy fruit, a dull, baked finish and little of its old freshness. The bottle often gives it away first. Wine seeping past the cork, a sticky or stained capsule and a cork pushed up above the rim of the neck all show that the wine has expanded in the heat.

Heat damage cannot be undone. Storing wine in Hong Kong covers how to avoid it in the first place.

The other faults you may meet

• Reduction. Struck match, rotten egg or cooked cabbage, from sulphur compounds formed during fermentation that linger when a wine has had too little oxygen. It can appear under any closure, screw caps included. It often fades with air, so swirl and wait; if that is not enough, decant the wine and give it time. A faint struck-match or smoky note can be deliberate, adding complexity; rotten egg is always a fault. An acrid, burnt-match smell can also mean too much sulphur dioxide, the preservative used in almost every wine and most generously in sweet whites.
• Light strike. White, sparkling and rosé wines in clear glass can pick up a smell of cooked cabbage or wet wool after time under bright shop lights or in sunlight. Reds are much less vulnerable and coloured glass protects far better than clear, so buy pale wines in clear bottles from shops that keep them out of bright light.
• Volatile acidity. A smell of vinegar from acetic acid, often with the sharp, nail-varnish-remover note of ethyl acetate alongside. Bacteria and some yeasts produce it. A trace passes unnoticed and some winemakers accept a little for complexity; too much leaves the wine sour and thin.
• Brettanomyces. A yeast, found most often in red wines, that brings farmyard, horsey and sticking-plaster smells, sometimes with smoke and clove. Opinions differ on a trace, which some drinkers accept as rustic character. When it smothers the fruit, it is a fault by anyone's standard.
• Unwanted refermentation. Unexpected fizz in a still wine, often with a haze, usually means fermentation has restarted in the bottle. A faint prickle in some young whites and rosés is deliberate, a touch of dissolved gas left for freshness. A fizzing still red is another matter.

What is not a fault

Plenty of sound bottles are sent back for the wrong reasons. All of these are harmless:

• Tartrate crystals. Small, glassy crystals on the cork or at the bottom of the bottle, formed from the wine's natural tartaric acid, often after it has been chilled.
• Sediment. Normal in older reds and vintage Port. Stand the bottle upright for a day or so, then pour gently or decant.
• Pieces of cork. A crumbling cork is a nuisance and nothing more. "Corked" means tainted, usually by TCA, so fish the pieces out and carry on.
• A closed wine. A young wine going through a quiet phase can seem mute and hard. Air may help a little; time helps more.
• A wine you do not like. A heavily oaked Chardonnay or a tannic young Barolo may not be your style. That is a question of taste.

What to do about a faulty bottle

In a restaurant. Taste as soon as the wine is poured. If something seems wrong, say so quietly and describe what you notice: "I think this may be corked." A good sommelier will check it; a corked bottle should be replaced. Speak up at once, because a complaint after half the bottle has gone is much harder to make.

The tasting pour is for checking that the wine is sound, not for deciding whether you like it.

If you brought the bottle yourself, the restaurant cannot replace it from its own cellar. Bring a spare for anything old or precious; corkage in Hong Kong covers the rest of the etiquette. It is also why our blind tastings carry a spare bottle for every flight.

At home. Put the cork back in and keep the bottle with most of the wine still in it, along with the receipt. Then talk to the merchant: a reputable one will want to know. With old bottles, some losses come with the territory, which is one more reason to buy more than one bottle of anything you care about.

When you host. Open and taste every bottle before it is poured. Keep a spare of the main wine. A faulty bottle caught in the kitchen is a non-event; one discovered at the table interrupts the evening.

Train your nose while the stakes are low. The first check when tasting any wine step by step is whether it smells clean. Make it for every glass, trust the first sniff and say something when a wine seems wrong. Nobody minds a guest who spots a corked bottle. Everyone minds drinking one.