It’s always good to just ask them before they start with you

With regards to physical limitations with your students, it’s always good to just ask them before they start with you (as an instructor) if they have any physical limitations you should know about. Then, adjust the lessons to adapt to those limitations accordingly. There are tests you can do to determine flexibility for example though. The best test I can think of is to have your student try to touch their toes with their legs straight and feet together. If they can touch their toes or come close they’re probably pretty flexible and most likely won’t have any problem swinging the club.