If you use your Medicare card when you see your doctor (you are not enrolled in a Medicare Advantage plan), you can set up an account on mymedicare.gov. This account will allow you to see what Medicare has paid for your medical services.
I was talking recently to a woman who said Medicare rejected a claim (did not pay for the service) and I told her the doctor might have used the wrong billing code. I told her she needs to get the billing code so she can try to find out why Medicare rejected the claim.
She can see the billing code and all of her medical claims by setting up an account on mymedicare.gov.
The screenshot below shows the home page for mymedicare.gov and the arrow shows where to start by creating an account.