Another option would be to charge them a monthly fee (rent). It can based either on the amount of hours they work each week/month or a flat fee no mater how many hours they spend training clients. This way they will have a better idea of how much they make each month and the same goes for you. All they have to provide is their certification credentials and a copy of their insurance. Of course the other options presented by others here are excellent alternatives as well. It all comes down as to how you want this to be handled.
Good point regarding knowing if the percentage is accurate/honest. The private gym I worked for paid me as an independent contractor. I was paid 50% or 60% of what the client paid. These were clients the gym provided to me.
Some gyms have trainers pay a flat fee/month and others allow trainers to pay a per client fee.
I guess it’s a matter of crunching the numbers based on meeting your overhead.