Is there a test method of proving a person being color-blind, without letting the test subject know, that he/she is being tested?
E.g. showing the person cards with colored dots like depicted here is not a valid answer.
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Depending on the instructions that you give to the person, you might be able to disguise the fact that they are completing a color-blindness test. You could use stimuli like those in the Ishihara test, but instead ask the subject to do a math problem where they add the previous two numbers. You can also tell them that sometimes no number will be presented, and have a few cases of actual blank tiles mixed in with the regular stimuli. If a subject reports the wrong sum consistently for trials that involve a particular hue, you can be reasonably confident that they are colorblind for that hue.
There is also a short test developed by researchers at City University London which has subjects track a square during a movie. It's probably no more subtle than the Ishihara test, but it is less well known and therefore subjects might not infer what the purpose is. See http://www.city.ac.uk/health/research/centre-for-applied-vision-research/a-new-web-based-colour-vision-test.