Fitness instructors offer the promise of a great body and typically use their own physique as evidence that the system works.
Read the biography carefully. Many of these people are gymnasts, ballerinas, or have a long-term background in yoga. They may be genetically predisposed towards a lean physique.
How many of them had great bodies to start with? What proof do you have that their body is the result of the offered system?
Apply these questions to exercise classes too.