A subtle test for color-blindness
I am colorblind, but I did not know that before I was 22 (!!). I got to know it by doing the Ishihara test. There are other tests you can do to test for colorblindness, such as the reverse colorblindness test (http://www.cs.unm.edu/~aaron/creative/colorTest.htm), where a "normal vision" person will not see the two patterns shown there.

Anyway, there is no test where you cannot ask "what do you see?" to the subject. On the other hand, I saw a photo where they were showing a northern light in both colorblind and normal vision (https://www.facebook.com/nlblog/photos/pb.236481989828593.-2207520000.1415264974./416457851831005/?type=1&amp;theater) -I hope facebook links are OK in this site-, where I see no difference between the two photos. You could for example show that photo pair to your test subject and ask a set of questions (e.g. which one do you like more?). If he/she is colorblind, then the answer will be "they are the same", so you have a strong indication, or if they are able to point out differences, then they most likely have normal vision.

I could think of an experiment where you just show several photos with different contrasts/saturations, and then add the northern light photo (maybe the author has some other photos you could use), so they are not aware of the underlying question.

I hope this helps