It's simple. Take a 9-month pregnant woman and an adult man. Wait until the woman is in labor, then kick the man in the balls REALLY hard. Have him rate his pain. Once the woman is out of labor you, ask her to rate her pain. Then, a couple days later, punch them in the exact same part of the face with the exact same amount of force. Have them rate their pain. The comparison will help you ignore the possibility of one person exaggerating. Do this with many people and average it out so you can get rid of the fact that some people will exaggerate one but not the other. Whatever is rated higher hurts more. Boom.