Picture this: The sommelier pours the first taste of a recently opened bottle and stands expectantly, waiting for you to taste it and declare the bottle acceptable. But what are you really supposed to be tasting for? If the wine is well balanced? Too tart? Just right?
It turns out that what you are really supposed to do is quite simple in theory; and a little bit hard to do in practice. The primary thing you are doing is tasting to make sure the wine is acceptable for your guests and specifically, if the wine is flawed. Now, you don't have to analyze if the tannins are in balance or if the winemaker had a heavy hand with the oak. You are simply tasting to see if there is a wet dog in your glass.
The primary flaw you are looking for is if the wine is "corked." This happens only in bottles with corks, so if you have screw cap wine, you have no worries. The reason a bottle becomes corked is a chemical known as TCA (trichloroanisole) which is a compound that forms when a mold in the cork reacts with the cleaning agents used in the production of the cork. With modern techniques, this happens in about 2-5% bottles of wines with corks, or about a bottle every 2 cases. Odds are we have all had a glass of corked wine or two in our day. It can sometimes be subtle and not enough to be unacceptable.
When you smell the wine, you will smell wet dog, wet cardboard or wet basement. It is a musty dank smell that is a little off-putting. This is different than the earthy barnyard scent you pick up in old world reds. Some corked wines will present obvious musty notes and others are a little subtle and possibly drinkable, although you shouldn't. I will usually taste a corked wine to be sure it has that same mustiness on the palate - my sacrifice for my friends at the table
If you think a wine is corked, you should comment gently to the Sommelier that you think the wine might be corked and invite them to taste. Here is where their training comes in. In an excellent restaurant, they will whisk away the bottle and taste it in private, perhaps with another Sommelier. They will not object to your analysis.
If the wine is in fact corked, they will offer another bottle of the same or an alternative similar bottle. You shouldn't have any fear in taking another of the same as TCA can vary from bottle to bottle and a single bottle in a case can be corked and the rest can be just fine. They will usually return the bottle to the distributor for a refund as this flaw is part of the wine business and there is an anticipated rate of return.
When the new bottle comes to the table, the Sommelier will usually offer to taste the wine to ensure this second bottle is not corked. Let them do it as they are your partner in getting an acceptable bottle to your guests. Additionally, make sure they replace your glass since the original glass is now contaminated with TCA that can only be removed with cleaning. If your table carries on to a second or third bottle you should taste each of those before pouring as well. TCA varies by bottle, so just because one was acceptable, they next one could be okay or not - it is random.
If the wine is not corked in their analysis, they will usually sell the bottle by the glass from the bar. This way, they can move the unused wine and frankly, make more money on the bottle! Don't worry about declaring a wine corked - the restaurant will be just fine.
In a poor restaurant, the Sommelier will fight your analysis and declare the wine acceptable if they find it without flaw. Stick to your guns. This is all about your pleasure and the pleasure of your guests. Insist on a replacement or an alternate bottle. If the Sommelier is unreasonable, escalate to the manager of the restaurant and they will almost certainly make it right for you.
Next time you are offered to taste the wine, swirl, smell and taste. If there isn't a musty wet dog in the glass, declare it acceptable and have the sommelier pour for the table.