One of the most common wine questions I get is, "How do you tell if a bottle of a wine that you got at a restaurant is bad? I never know when to send one back." Let me start by saying what does not constitute a bad bottle. A bottle is not bad just because you don't like the wine. There are many variations in wine-making style, so a bottle that doesn't suit your preferences is not necessarily defective. Of course, the sommelier should help you select a bottle that's to your liking, but ultimately only you are responsible for your personal tastes.

A bottle is not bad just because the label is damaged. Most wine travels thousands of miles to get to you, and there are plenty of opportunities for bumping and grinding. Likewise, in a cellar where thousands of bottles are stored together, one bottle can break, leaking wine onto hundreds of others. This does not affect the wine inside the intact bottles.

A bottle is not bad just because it has little white crystals accumulated at the bottom or adhering to the cork. These crystals (called tartrate) are a natural by-product of unfiltered, unprocessed fine wines and are totally harmless.

A bottle is not "corked" just because it has bits of cork in it (all this means is that an inexperienced waiter pushed the corkscrew all the way through the cork, thus forcing pieces into the wine) or because it has an unsightly or even moldy cork. The term corked has a very specific meaning, which I'll explain in a moment. There are essentially four things that constitute defects in a bottle of wine such that you should send it back: It can be corked, oxidized, maderized or refermented. Corked

Corks are natural products, and some microorganisms like to eat them. A wine is properly said to be corked when it has come in contact with a contaminated cork during the aging process. The results of this contamination are almost always unmistakable: The wine will smell like a wet basement after a flood or dirty socks left in the hamper a little too long: moldy, nasty and not at all enticing to the taster. On the palate, it will be astringent, lacking in fruit, with a raspy finish. Sometimes you may even notice a paint-thinner quality. Still, when you catch a wine in the earliest stages of being corked, there may be some doubt -- here, all I can say is that the more you taste wine, the higher sensitivity you will attain in identifying corked wines. Also, if a wine is served too cold, you may not catch the telltale aromas on the initial offering. This isn't your fault, and you are still well within your rights to send the bottle back once the defect becomes clear. You cannot, however, discover a corked wine by smelling the cork. Many fine wines have issued from bottles with funky-smelling corks, and vice versa. Oxidized

Oxygen is wine's invisible enemy, and when a wine gets exposed to air, it becomes "oxidized." The result is flat, lifeless wine that loses its pretty, vibrant fruit scents and tastes insipid -- it will likely remind you of vinegar. The trained eye will also often notice a certain dullness in the color. In whites, it can be light to dark yellow or even brownish. It is much less obvious in red wines. Maderized

Heat is another destructive force exerted on wine, usually as a result of bad storage. When one says a wine is "maderized," it has been literally baked (this often happens in the holds of cargo ships as they cross the oceans in summertime). It actually tastes like Madeira and is reminiscent of almonds and candied fruits -- admirable qualities in dessert wines but unacceptable in dry wines. You may also notice, in the unopened bottle, that the cork is pushed partly out of the neck (due to expansion within). Refermented

Fine wine is a living thing, the product of controlled fermentation. Occasionally, some residual, dormant yeasts will wake up, and a wine will undergo a second fermentation after it has been released and shipped. This manifests itself as effervescence, or fizziness, on the tongue. Of course, this is desirable in champagne (which is purposely refermented in the bottle in order to create the bubbles), but never in fine still wine. It's difficult to learn to identify these flaws just by reading about them. Only experience and time will give you the training you need to spot every defect. But if you think a bottle is bad, ask the sommelier for confirmation. Don't be afraid -- at any reputable establishment the sommelier will not take a rejected bottle personally (not that you should care if he or she does). It is, after all, a statistical certainty that a certain percentage of wines will go bad through no fault of the restaurant. Some of these we can return to our distributors for credit; with others we'll just take the loss. We compute our wine prices with the expectation that we'll get some bad bottles. If we honestly think you're crazy and believe with all our hearts that the bottle is perfect, we'll still take it back -- and maybe later have a glass ourselves.