This all depends on your scenery.
You know when, in a game, a player says "I'm going to move into the woods but stay far enough back so you can't see me", or, "I'm going to move to the edge of the wall so that I can fire at your guys"? That's the approach we take.
If two enemy are within a section of wood then they can see each other. Obviously apply a scenario rule if you have a gigantic wood!