How can I know if a gameObject is 'seen' by a particular camera?

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There's a whole slew of ways to achieve this, depending on the precise functionality you need.

You can get events when an object is visible within a certain camera, and when it enters or leaves, using these functions:

OnWillRenderObject, Renderer.isVisible, Renderer.OnBecameVisible, and OnBecameInvisible

Or you can calculate whether an object's bounding box falls within the camera's view frustum, using these two functions:

GeometryUtility.CalculateFrustumPlanes GeometryUtility.TestPlanesAABB

If you've already determined that the object is within the camera's view frustum, you could cast a ray from the camera to the object in question, to see whether there are any objects blocking its view. Physics.Raycast

knowing whether an object is in the camera frame of view
How to convert measure in pixel to 3D space units ?

Is it just me, or does the example for http://unity3d.com/support/documentation/ScriptReference/GeometryUtility.TestPlanesAABB.html only work for a static camera? It gets the frustum planes in the Start() method, so if the camera moves it will be inaccurate. Am I correct in thinking that?

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@Gillissie (several years late): I got this to work just by moving that line into the Update method.

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Note: The "GeometryUtility.TestPlanesAABB" seems to return true even if just a "portion" of the GO is visible. I would not use this if you want to ensure the entire GO is visible.

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Renderer.isVisible does not exist

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there is a problem with OnWillRenderObject, Renderer.isVisible, Renderer.OnBecameVisible, and OnBecameInvisible , it consider rendering even if object is just enable in scene.

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