Only raise the legs if someone gets choked out.
A pneumothorax or lung collapse is usually identified, without the aid of a X-Ray, by difficulty breathing, diminished lung sounds (you'd need a stethoscope), and unequal chest rise. In terms of a tension pnuemothorax you can add tracheal deviation, and respiratory distress. The second set of signs means it's really bad and that's usually when you'll really know if the lung is collapsed. Basic rule is anyone having continuous difficulty breathing after taking a hit to the chest should be checked out at the hospital.
Treating a lung collapse is getting into things that should be over the lay-person's head. If someone's lung collapses dial 911 immediately! Place them ON TO their injured side to allow the good lung to fully expand or the position of comfort. They are going to look like absolute shit. Just wait for the medics.
DO NOT start smacking people.