Remember, this only applies to Original Medicare. If the pt has Medicare Advantage (which 42% of medicare beneficiaries do), it is as if they don't have medicare at all, and you can charge them cash. Also some pts only have Medicare Part A (for inpatient), if they don't have part B, you can charge cash.
Some additional things you can do to protect yourself include:
1. get a copy of front and back of pt's primary (and secondary if they have one) insurance
2. do not give superbills to pts who don't provide this information. They can't submit bills to medicare if they don't have them
3. Have pts sign an agreement that they do not have original medicare, that they will notify you if this changes, that they will need to find another physician if they do, and that failure to report this is grounds for termination from the practice.
4. otherwise if pt is on SSDI, over 65, has ESRD or ALS you should assume they have original medicare until proven otherwise