Stand in natural light, push up your sleeves, look at your veins. What color are they? If they're blue, you're cool. If they're green, you're warm. Can't tell? You're neutral. 

Look in a mirror and hold a piece of gold jewelry next to your face.
Now do the same with a piece of silver. Which one makes you shine?
Silver = Cool
Gold = Warm
Can't tell? Neutral

Do you feel at your best in bright white and true black? Cool. Do you gravitate toward off-white and browns? Warm. Feel beautiful in both? Neutral.