Real Italian Parmesan, or Parmigiano-Reggiano, is expensive and it's expensive for a reason. It is not an industrial product, it is aged, and it comes from a strictly delimited area in Northern Italy with centuries of experience, giving it the DOP mark (denominazione di origine protetta).You will find plenty of attempts out there to pass off imitations for the real deal.
Real Parmigiano-Reggiano will have the characteristic branding on the rind that you can see above. Even small pieces should have this branding, although the top of the form is without it. The official Consortium in Parma has told me that the cheese should be cut by the vendor in the manner shown in the figure on this page from their site. This allows even pieces from the top to show the branding.
