Your frustrum is defined by six clipping planes (near/far/left/right/top/bottom). Dot product of a point and plane tells in which side of the plane point is. If one or more of these corners is inside the frustrum, it's atleast partially visible (unless frustrum is reasonably bigger than your boxes). Search for topics and you'll get a lot of topics conserning frustrum culling and plane equitations. There are much great topics. So did I too.