Frank's organization operates in a shared responsibility cloud environment. What can Frank do to determine if he is suffering from a denial-of-service (DoS) attack against his cloud hosting environment?

Frank's best option is to review the anti-denial-of-service and other security tools that his cloud hosting provider provides and to make appropriate use of them. The major infrastructure-as-a-service (IaaS) providers have a variety of security tools that can help both detect and prevent DoS attacks from taking down sites that are hosted in their infrastructure. Calling the cloud service provider's ISP will not work because the ISP works with the cloud provider, not with Frank! It is possible the cloud service provider might be able to assist Frank, but they are most likely to instruct him to use the existing tools that they already provide.