Depends upon age. Obviously inability to distinguish between red / green colors. Some have trouble with blue/yellow. It does assume the child knows his colors. I am not aware of a condition where no colors are seen. These are usually specific to certain colors.
COLOUR CHARTS. You show them a colour chart and ask them to name the colour. It is a simple test.
Color plates. Your pcp may have a copy of the ishihara plates that are used as a screening test for the various levels of color blindness. I seldom have a request for this screening but find the process helpful. My pcp found a mild issue with my vision with my pre college exam & I keep a copy of the book around to screen kids if asked. If he is old enough to answer questions this can pick it up.
No need to. It's a simple test where they show the child a book (called an ishihara) with numbers hidden within colored dots. He/she will be asked to identify the numbers and based on that the doc will be able to tell if he/she is color blind.
Generally teen years. The color vision of someone with color deficiency is natural to them until they start to compare their color vision with others. This can occur in later childhood but usually occurs in the teen years.