If you can see veins on your inner wrist, you may be able to use their color to identify your undertone. Greener veins indicate a warm undertone, while blue or purple indicates a cooler undertone. If your veins look green and blue, you most likely have a neutral undertone.
Foundation and Creme Revealer shades are named after their product, undertone and shade depth. For example:
RN2 = Refresh Foundation, neutral undertone, shade level 2
CW3 = Creme Revealer, warm undertone, shade level 3
UC2 = Uplift Foundation, cool undertone, shade level 2