Most people who tan easily have warm skin undertone, or if you burn easily and tan very minimally, you are most likely to have cool skin undertone. If you feel you are somewhere in the middle, you likely have neutral skin undertone.
Another quick way is to look at the colour of your veins. Do they look slightly green? If so you’re warm. If they have more of a blue or purple tint your undertones are cool. If you feel they are a blue-green colour then you are more neutral.
With a fresh face (no makeup on) hold up a piece of clothing or material that is pure white then one that’s off white. Which suits you better? If it’s the pure white your undertones are probably warm. If you looked washed out in pure white, and better with the off white, then your undertone is probably cool. If your skin looks good in both, your skin tones are more neutral.
Do you tend to wear more gold or silver jewellery? If you think you look better in gold jewellery you have a warm undertone. If silver is more flattering your undertone is cool. If you love both you may have more neutral undertone.