This assumes you have already created a static cluster.
The cluster will no longer be listed at Servers > Clusters > WebSphere application server clusters, and will instead be listed at Servers > Clusters > Dynamic clusters.
At Servers > Server types > WebSphere application servers, the servers in the cluster will be listed. In this example, server01 is not part of a cluster, and server03 and server04 are members of cluster1. This view also can be used to know if the JVMs are in a vertical or horizontal cluster. In this example, server03 and server04 are in a horizontal cluster, as the servers are on different servers (was1 / was2).
Servers > Clusters > Dynamic clusters > cluster_name > Dynamic cluster members will display the cluster members.