how we know that the person …

Colour (color) blindness (colour vision deficiency, or CVD) affects approximately 1 in 12 men (8%) and 1 in 200 women in the world. In Britain this means that there are approximately 2.7 million colour blind people (about 4.5% of the entire population), most of whom are male.
There are different causes of colour blindness. For the vast majority of people with deficient colour vision the condition is genetic and has been inherited from their mother, although some people become colour blind as a result of other diseases such as diabetes and multiple sclerosis or they acquire the condition over time due to the aging process, medication etc.
Most colour blind people are able to see things as clearly as other people but they are unable to fully ‘see’ red, green or blue light. There are different types of colour blindness and there are extremely rare cases where people are unable to see any colour at all.