Ask them how much they weigh. Women that are self-conscious will deflect the question, respond with aggresion, or throw their drink in your face.
Women that are truly confident about their bodies will tell you the number, even if its some embarassingly huge number. But in my experience, I haven’t gotten a straight answer from any woman who ended up weighing more than 160 lbs. It says a lot about their hangups and problems in life.