It never ceases to amaze me how many people fill their wine glass to the top. This is not a rant just an observation that is a shame not to be able to swirl the wine in the glass and enjoy the aromas of the wine as the smell is 90% of the taste.
The first sense you use when assessing a wine is sight, the colour of the liquid in the glass can give you an indication of age and condition of the wine. If the wine is a Caberent Sauvignon it will be dark and opaque and if it is a Pinot Noir or Gamay for example it will be lighter in colour. If it is an older vintage it will have different shades from rim to core. Red wines lose colour with age while white wines gain colour with age.
Some wine makers do not fine or filter their wines so you may experience a few tartrate crystals floating in the bottle as you pour, while these may look off putting they will not do you or the wine any harm.

Checking the colour	Tartrate Crystals

Then you should fill the glass no more that two thirds full and swirl the liquid gently in the glass and take a good sniff. This gets the olfactory cells going and sends messages to the brain, if you taste a lot you will have an idea of what the wine is. If not you can still ascertain plenty from the aroma, is it an appealing aroma, do you want to go on and taste? Hopefully it will smell pleasing, of fruit, and possibly even minerals and spice and maybe even grapes! You should feel enticed to taste.
If it smells of wet cardboard or musty there is a chance it may be corked. If it smells like your Granny’s sherry that has been left open for weeks it may be oxidised. While this is rare if it does happen you should be able to bring it back to your reputable wine merchant.