In Whimsical you can be a member of multiple workspaces (2 workspaces on the Free plan, unlimited workspaces on paid plans) this article explains how to switch between them.
The name of the workspace you're currently in is shown in the top left corner, above the sidebar:
To switch over to another workspace, click your current workspace name in the top left corner, then hover over Switch workspace and select it from the list:
You'll see which workspace you're currently in marked with a green dot. 🟢
You'll also see under each workspace name the total number of members, and the plan the workspace is on: