The plate is half black and half white
Suppose a disc rotates like a record on a record player. The plate is half black and half white. Suppose you have an unlimited number of color sensors. How many color sensors do you need around the disk to determine the direction of rotation? Where should they be placed?
The answer is 2.
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1 solution
We can place 2 color sensors right next to each other (actually you can place them however you want except directly opposite each other, but it's easier to understand then).
Then, we wait until both of them sense the black half of the plate (or analogously the white part). From this situation, we wait for one sensor to see the white (or black) part again and call the first sensor to see this A and the other one B. Now, we can be sure that the plate is rotating from A to B.