Smell a wine for wet cardboard and mold. If you smell these distinct flavors, you'll know because your nose will wrinkle and you'll sharply pull your nose away from the glass. This is a clear indication that the wine has not been cared for and that the cork may have tainted its intended flavor.
Another indication to watch for when first inhaling the bouquet is the smell of vinegar. Frequently a wine that has been left open and not vacuum sealed will turn, and this will spoil the wine. Wine wants to be enjoyed 30 minutes to 5 hours after being opened and aerated.
Most of your average restaurants will offer you a sip of wine before you decide on purchasing the bottle. Don't be shy! If you notice that the wine smells like dirty sweat socks, politely tell the waiter that the wine isn't what you had hoped for. There is no need to be rude to a waiter who may or may not be the bench warmer at your local university.
Please be aware that turning down the wine will often call the attention of the manager. Be confident in your assements as you are always the best judge of wine for your meal. I personally have told a manager that the wine had turned and was too moldy in bouquet. This wine would certainly not suffice for a meeting of nobles, and therefore it should not be served to me.
The best wine you can drink at any meal is your favorite wine.