The three fastest tests
1) Vein test
Use daylight and look at your wrist veins. Green suggests warm, blue/purple suggests cool, and mixed suggests neutral.
2) Jewelry test
Hold gold and silver jewelry close to your face. The metal that makes your skin look more even usually matches your undertone.
3) White vs cream test
Hold a pure white and a cream fabric near your face. White favors cool undertones; cream favors warm. If both are fine, you're likely neutral.
Bonus: Lipstick test
If you have both a warm coral lipstick and a cool berry lipstick, compare which looks more natural on your lips and cheeks. That often reveals undertone quickly.