True beliefs reliably control anticipation. If you build a sky-detector that bases its decisions on color discrimination, you should anticipate that the detector's decisions will be appropriate to the extent that your theory of the sky's color is correct.
Let's imagine that the sky-detector's function is to orient a water spray toward the sky, so as to maximize the area sprayed. (Because "sky" is what's up when outdoors.)
Place the device on various surfaces. The color of sky is the color of whatever makes the device most confused, spraying in random directions.
While you make an interesting case for testing my belief, I do not know how to build a "sky-detector". So I still am oblivious as to whether my belief is true or not.
Let's say that I believe that the sky is green.
1) How can I know whether this belief is true?
2) How can I assign a probability to it to test its degree of truthfulness?
3) How can I update this belief?
Thank you.