Imagine a real battlefield with its contours, morning mists and haze of dust. Towering over our miniature battlefield we are unaware of all this, but the troops represented by our models would not be so lucky. Just as their real life counterparts cannot see through hills or hedges, we must assume that our models cannot see behind corresponding terrain features.
As it is impossible for us to say exactly what everyone's terrain looks like, it is not practical to be definitive about which kinds of terrain block line of sight. You must be prepared to use your own judgement within the following guidelines.
The easiest way of checking what a model can see is to get down over the table and take a model's eye view, but be reasonable about this, as in reality it would be much more difficult to see enemy troops than over a perfectly flat, mist-free gaming table.
It is only possible to see through up to 2" of woodland. So, if a model inside a wood is within 2" of the edge, he can see out and he can also be seen. If it is further than 2" inside the wood, a model can neither be seen by models outside nor can it see them. If both the model and its target are inside the wood, then the line of sight is reduced to the farthest they can see - which is 2".
Normally in Warhammer, woods are represented by a few trees stuck on a base. It may happen that it is possible to see a model/unit on the other side of a wood because there is some space between the trees. For simplicity's sake, a degree of abstraction is required: players should always assume that the entire base of the wood is covered in trees as tall as the tallest tree on the base, and therefore the entire area marked by the wood's base blocks line of sight.