That depends. There are similarity relations between colors: for instance, green is more like blue than it is like red, orange is more like red than it is like purple, etc. So if someone said that to them, blood looks more like the color of leaves than like the color of an orange (the fruit), you would know that they are seeing colors differently from you. It is usually thought, however, that if someone were born with their color experience completely inverted along the whole spectrum (so that red becomes violet, orange becomes indigo, yellow becomes blue, etc.) then the similarity relations would be preserved, and there would be no way to tell. (This thought experiment, the “inverted spectrum” goes back at least to John Locke in the 18th century.) Some argue, however, that the detailed similarity judgements that people make about colors are not really as symmetrical as that scenario would require, so that, in fact, detailed questioning about similarities would always reveal that something was different, even if it were not immediately obvious.
However, the medical conditions that other people are talking about in this thread all, I think, lead to defects of color vision, such that people who suffer from them will not be able to distinguish colors that most of us will see as clearly different: a sort of acquired color blindness. Cheshire Human’s mother, it appears, can no longer distinguish certain greens from either blue or gray, for example. It is easy enough, in principle, to detect color-blindness, even congenital color-blindness, in yourself or others. You just have to notice that certain things that most people say are different colors look the same to you.
I say “in principle,” however, because, in practice, most forms of innate color blindness are not very noticeable.* Indeed, there is no evidence that anybody throughout the course of human history ever realized that color blindness existed until John Dalton (in the 18th century again) realized that he could not distinguish certain colors that other people could (and in fact it appears that Dalton suffered from a rare form of color blindness considerably more severe, so more noticeable, than more common types). In practice, someone who loses some of their ability to distinguish colors due to disease, is far more likely to notice the defect than someone born color-blind.
*There is a *very* rare form of congenital color blindness, [achromatopsia](http://consc.net/misc/achromat.html), in which the sufferer has no functioning cone cells at all, only rods, and thus cannot distinguish colors at all, only shades of relative light or dark. One might think that no-one could fail to notice this. However, a person with achromatopsia has far worse, and far more noticeable, problems than not being able to distinguish colors. They are effectively unable to see at all in ordinary daylight (which overloads rod cells) and only have useful visual function in dim light. Even then, they cannot see fine detail (which, like color vision, depends on the cones).
