Through automatically checking whether cloud services satisfy customers' requirements, cloud service certification promises cloud providers competitive advantages, e.g. by attracting new customers. However, certification can increase costs of cloud providers, creating incentives for fraudulent providers to save costs by only pretending to satisfy customers' requirements. Opportunistic providers are fraudulent providers who will only cheat if they are not caught. In this paper, we propose an approach to support cloud service certification of opportunistic providers. To that end, we introduce a method to model the behavior of opportunistic providers and propose a framework supporting test-based certification which builds on randomized testing and is non-invasive. We show how our framework reduces the willingness of opportunistic providers to cheat, and present experimental results of tests supporting the certification of requirements related to resource availability, resource provisioning, and quality of service.