The useful console command point file comes into play when troubleshooting visibility leaks. What point file dose, is shows you a dotted line from an entity in the map, and will go through a wall, roof, ground, or such. Most of the time the leek is cause where the point file goes through the wall, though entities such as INFO_NODES out side the map will cause the leek to be appear because half life can not link that entity to the inside of the world.
You can break visibility between rooms most certainly using a corridor with a pair of 90 degree turns in it. A U turn should totally cut visibility, and an S turn with the proper length of corridor in between the two turns will almost always work. Below that, controlling visibility becomes an art. Often, you will be amazed by what the engine decides you can see from a given point. Itís important to carefully walk your level and watch polygon counts for this reason. You can also use hint brushes though I won 't tell you how to use them, as they are very complex, and I don 't get how they full work yet.
Note: If you haven 't done a Full Optimisation compile on your level, you cannot rely on the visibility or poly count results you obtain. Full Optimisation cuts down counts far, far more than a Fast Approximation does. Similarly, if you don 't compile with optimisation turned on at all, your whole level is visible from all points, leading to counts that are much higher than they will be after optimisation.
In order to increase your chances of blocking off line of sight between areas, there are several things you can do. First, make use of kinks in between areas as described. Second, make sure that walls extend from the floor of a room to its ceiling. Third, make sure your walls fit snugly together. Fourth, keep the number of connecting doors, windows or vents between rooms as low as is reasonable so that you can more easily troubleshoot. Fifth, treat your outdoor areas in the same fashion, dipping your sky brushes down like a ceiling to close off one region of the outside from others. Use hills, hedges and tall buildings as walls to reach up and touch these sky brushes, thus completing the seal.