How do i detect if object is visible from camera.

Basically, i need a pretty basic vision detection. Camera sees the object from behind, but i need to know if there are any walls blocking the object. How do i fire rays at this object at parts the camera sees? No, firing at tanks pivot point is pretty awful way to do it, as it doesnt work if the center of tank is blocked by anything.

just detect if object is on the cameras viewport Vertical coordinates 0 (center) and fire ray there, if ray hits a wall, move slightly up, fire again and so on. But for performance reasons i dont want to bombard entire screen with rays, just the region where the tank is actually located.