You can start with our "Instant color blind test". You'll get a feeling of whether you might have some troubles with color.
This is however not a medical diagnosis. If you are looking for a more "medical approach", try the following steps:
Perform a simple Ishihara test online
The test consists of colored plates, made of apparently random dots in color and size. Within the pattern are dots which form a number or shape clearly visible to those with normal color vision, and invisible, or difficult to see, to those with a red-green color vision defect, or the other way around.
Dots are actually based on a limited number of hues, appearing in different sizes, hues or saturation. The main shape is made of colors belonging to group of similar hues (e.g. orangish colors), while the background part of the plate is made of colors belonging to another group of colors (e.g. greenish colors).
These colors are selected on confusion lines specific to the color blindness being tested, so that if you are colorblind, they seem to belong to the same hues. And thus it is impossible or quite difficult to see the shape.
Explore further your color vision with Farnsworth-Munsell 100 Hue Test
It tests your ability to arrange small differences in various color targets with constant lightness and saturation. If you are colorblind, you will usually be able to arrange part of the color targets perfectly, while other areas will be astonishingly quite difficult. For instance, if you are green weak, you will be most challenged in the brown / brown yellowish area (end of first row) and blue cyan area (end of 3rd row). Give it a try Farnsworth-Munsell 100 Hue Test. And if you need to improve your score, you might want to try our ColorCompass :). It's always easier when you know what hue you are working with.