The sommelier will show you the unopened bottle. This is your opportunity to ensure that this is the exact bottle you ordered. Check that the vintage is the one you asked for.
Once you have accepted the bottle, the sommelier will uncork it and might show you the cork. Look to see if the cork is in good condition, and if it looks suspicious, smell the cork for mustiness. You can usually tell more by smelling the wine when poured.
The sommelier will pour a small amount of wine into the glass of the person who ordered it, or he or she may ask who would like to taste. If the table has ordered more than one wine, the sommelier may ask who would like to taste each bottle.