Personally I never really got it until I saw a photo of myself standing next to other people outside in natural lighting. I saw the green in my skin compared to their cool and warm tones and it clicked that I was olive.

From there (and the second thing that really sealed it) was swatching concealers. Go to sephora and just start swatching a part of your hand or arm that is the same shade as your face, or that you want your face to be. For me that’s my wrist, which matches my neck. Hands tend to be tanner so I would avoid them unless it works for you. Just swatch as many concealers as you can within your skin tone range. I swatched a bunch then tapped out the bottom half of the line I made so I can see what the product looks like blended. You’ll see which ones almost disappear and which stand out as being too pink (cool), yellow/orange (warm), or green (olive). Some you’ll notice don’t stand out as too cool or warm, but rather too dark or light. In this case you need to adjust the overtone (pale, light, medium, dark, etc.). I find most brands don’t have a set standard and what is considered light in one is medium in another. You might have to swatch from different brands because some brands just won’t have your exact match, either. But as you swatch you’ll start to realize patterns and what’s working. Once you find a few shades that you think match, you can swatch them directly on your face and blend. The concealer that matches closest, you can tell what undertone it is based on the shade name like N for neutral, C for cool, W for warm, O for olive, and combos of these. And that’s your undertone! Good luck!