renderer.isVisible

Yes! I don't know how often you'd want to check it, but if you want it per frame..

var objectToStalk : GameObject;
private var objectToStalkRenderer : Renderer; // Cache this so its faster

function Start()
{
 objectToStalkRenderer = objectToStalk.renderer; // This reduces a get_component call in the update loop
}

function Update()
{
  if (objectToStalkRenderer.isVisible) print ("Jessica Alba is hot.");
}