Appearance
When looking at a wine's appearance, it is best to pour just a little into the glass, tip the glass at an angle and look down through the glass. You'll notice on all wines that the colour is lighter at the rim. The width of this paler rim will tell you how concentrated the colour is. As a rule, the wider the rim, the lighter the wine.
If it's a red wine and has a very deep and concentrated colour, it's likely to be pretty deep and concentrated on the palate too. If it's less so, it might be a lighter style or could be a very mature wine. Mature wines will often develop more of a brick-like colour.
If it's a white wine, and the colour is golden and rich looking, the taste of the wine is likely to reflect this. Perhaps the gold is a result of oak aging. If it's pretty clear with a greenish tinge the wine is likely to be really bright and fresh.
One thing you don't really want in your glass is sediment. White wines should not have any and the same can be said for most red wines. The exceptions are mature wines and Port, where sediment is a good sign. However, you still don't want it in the glass so it's a good idea to carefully decant these wines before serving.