Hedges: both units are obscured if the hedge is between the units but neither is touching it. If one unit is touching the hedge it is in cover.
The top of a building: any unit within a terrain peice can claim cover. Remember any terrain peice that a unit can move through is considered cover.
Looking around a corner: if a model is partially obscured by the corner of a building, but is not touching that building, it is obscured. If it is touching the building it is in cover.
Seeing through cover: a wall 3" high but only 1/4" thick - the wall is in a battle zone, so has numerous small holes as well as windows and doorways. An infantry model is touching the wall, so can see through it and fire freely through it. Similarly any model targeting that unit may see it, although it will be in cover.
A more size related example: a woods terrain peice. If the tank is within 4" of the edge of the woods it can fire out and be fired at. If the infantryman is within 1" he may fire out and be fired upon. If the bothe the tank and the infantry model are within the woods they can see each other at a distance of 5".