The sensitivity and specificity of the somatotype and a body image questionnaire used to predict obesity were measured. This is a cross-sectional study with 294 college student’s participants from northern Mexico (143 women, between 18 and 35 years old). The somatotype was measured through anthropometry. The Gardner’s body image questionnaire was applied four times, including the following questions: How do I look? How do I want to look like? How do I think others see me? and How am I seen by others? Both the somatotype and the body image questionnaire correctly predicted between 90% and 95% of obese and nonobese cases; in addition, they showed 86% discriminative power to determine obesity. Endomorphy and ectomorphy, as well as the question How am I seen by others? showed the highest specificity (~90%). Both the somatotype and the Gardner’s body image questionnaire are two valid, sensitive, and specific instruments to determine the degree of obesity.