Generally speaking, if you look better in silver jewelry and you burn or get a rosy-red tan in the sun, you're probably a cool. If your skin looks more alive in gold jewelry or you tend to get a more golden tan, you're likely a warm. And of course, you're a neutral if you seem to be a combination of the two. Luckily, foundations are labeled accordingly, taking the majority of the guesswork out. Here's a hint: most people are in the warmer skin-tone category.
Don't Think Pink:
Unless you're really fair or cool, anything with pink undertones is probably not going to work. For something more natural looking, a yellow-based foundation is likely to be a universally flattering choice, as most skin has yellow undertones.
Sure, you can analyze your skin tone to your heart's delight, but the only true way to know if a foundation shade will work for you is to try it on. The best way to do this is to apply three different shades of foundation in vertical stripes, from the cheekbone to right below the jawbone. Avoid trying it on your arm, hand, or wrist, as these areas of the body can be different shades than your face. When in doubt, you can also sample colors on the forehead or décolletage.
Follow the Light:
Next, head to the window (or even outside) with a mirror, and check out your foundation stripes in natural light. The shade that disappears and blends into the face and neck is the one for you. Just keep in mind: the point of foundation is to even out tone, and not to change the color of the skin.