How can you tell if a position can be seen by the camera?

Use WorldToScreenPoint.

Description: Takes a 3D world point and returns the following: A Vector3 where the X/Y components map the position in screen coordinates the worldPoint maps to, and the Z component maps how far away from the screen plane the 3d position is (positive if in front of camera, negative if behind). A boolean which is true if the point is visible on the screen or not.

The last part is what you're looking for.