On-site Fitness Center & Proof of Health Insurance

[Guest calcu]

We have an on-site fitness center that is generally limited to employees with health insurance. We are considering opening up the entire center to all employees, whether they have our health insurance or not. If we do this, can we require the employee to demonstrate proof of health insurance (because they do not have our insurance)? We currently and will continue to require all employees wishing to participate to get a release signed from his/her PCP, but I was wondering whether we can go the extra step and require proof of health insurance from those employees not on our health insurance?

Thanks