Another way is to see what kind of jewelry or colors flatter you. If you think gold jewelry and earth tones look best on you, you likely have a warm undertone. If you prefer to wear silver jewelry and wintery colors, you probably have a cool undertone.
I, for example, have a warm undertone.
Cool skin tones look best in a blue-based, pinky or berry red.
Warm skin tones look best in orange, tomato or brown-based reds
Neutral skin tones can pretty much wear whatever colors they want.
All skin tones can wear a true red effortlessly.
This first group of reds above would suit those with a cool skin tone well due to their slight pink undertone.
The shades above would suit those with a warm undertone. These range from orangey red to brown-based red. The second shade from the left, M.A.C Red is a pretty true red and should look good on anyone.
This next group of colors is for the more daring. These are mostly warm-toned reds and would look good on neutral to warm skin tones.
Wearing a bold color like red usually calls for some reinforcement. Lining the lips prevents the color from bleeding and helps the lipstick to last longer on the lips.
Be sure to also exfoliate your lips before wearing bright colors.
Applying concealer on and around the lips helps the color stay truer and last longer.
My posts on How to Pull of Dark Lipstick and How to Make Lipstick Last All Day also applies to any bright color like red.
These are just guidelines. My motto is wear what works. If you see a color you like, wear it :-)
If you are planning on smooching anyone this Valentine's day, it might be best to stick to a lip stain or a nude color to avoid smudging his (or her) face with your perfect shade of red.
Hope this helps.
Thanks for reading :-)