A major challenge of interoperability projects and initiatives is to validate that different implementations work together and are compliant with underlying standard specifications. Interoperability testing can ensure that required end-to-end functionality is adequately fulfilled and all systems are implemented in conformity with existing standards. Adopters of standards have different methods to prove fulfillment of interoperability requirements. This often results in different efficiency, different quality and a lack of conformity. In this paper, we present the interoperability testing framework used in PEPPOL (Pan-European Public Procurement Online). The framework has supported the project well in establishing various production pilots that interoperate together in the field of e-procurement. It thereby aggregates different testing perspectives ranging from conformance testing to compliance testing and provides guidelines how to prove implementations through testing scenarios. Consequently, the key research question in this paper is how interoperability initiatives can prove that different implementations are compliant with underlying standard specifications and how interoperability can be ensured among different implementations beyond the technical approval mechanisms provided through conformance testing.