You can certainly look on the medicare website to see what their billing guidelines are, but there are some things for the physician to consider:
The non-participating physicians (non-PAR) can decide on a per-patient basis to accept the reduced fee (accept "assignment") or balance-bill patients up to 15% more. Therefore, the non-PAR physician that balance-bills every patient, may receive up to 9.25% more than a PAR doctor for the same services. The downside of non-PAR is that Medicare reimburses the patient directly and the physician must invoice the patient for the full amount: the payment, co-payment and balance-billing charge. If the physician postpones invoicing the patient until the patient receives reimbursement from the insurance, the doctor risks collection problems with financially irresponsible patients. Hope this helps you out a little bit.