   Line of Sight
  So im makign a line of sight AI for when a player character is in the line of sight of a guard it starts to chase you. Is ray casting the way to go for checking for Obstacles in the way? Ive got the distance and feild of view parts okay i think. Im just not sur ewhat the best method for checking for obstacles. Another way is to get the direction from the guard to the player and send out a ray in that direction to check for obstacles. Thats not real ray casting and i was just wondering whether people think that would suffice or i should go for the proper way?   
  You need some form of collision query, definitely.  It's sometimes called ray testing/checking.  You can use most physics engine for these kinds of things, including Bullet or ODE.You could just do a simple implementation by checking ray to obstacle bounding box intersections, but that wouldn't be too accurate.;  What you want is a Bounding Frustum. Basically, you take your camera position and an occlusion box and make a plane for each side of the occlusion box; each plane intersects all the other planes at the position of your camera.Anything inside that occlusion frustum is not visible and can be ignored. You can mix that with portals to further speed things along. Also, you can instead use visibility frustums - This is largely a design choice dependent on how "open" your model space is.Add in an octtree and you should have no speed problems for most things.(Also, your LookAt matrix * your View Matrix is, essentially, AFAIK, a visibility frustum)   