With names like "Killer X," "Brazilian Butt Lift," or "Enlightened Yoga," it's not always easy to gauge what a fitness class might be like. Class descriptions are often vague and nonspecific, and there's no real way to get a sense of the instructor until you see him or her in action. (And, more often than not, it's the instructor that sets the tone of the class.) Maybe the classroom is small, the lighting is off, or the air is too stuffy — all things you are unaware of until you're there.
Rather than being in a situation where you find yourself walking out of class, observe it from a distance first. This gives you a chance to see what the class is like, but without having to commit to the hour-long sweat session. Once the class is at least 10 minutes in, stand outside the door and casually watch both the instructors and students. From your observations you should be able to tell what the class pace and the instructor's style is like and if it's enough — or too much — of a challenge.