WetGeek
It's down to the theme you apply. For example, the plata-lumine theme variants will show in the workspace widget: a gray bar for inactive workspaces and a highlighted bar denoting the current workspace.
See here:
I've seen dividing lines between the workspaces on one theme and the switcher was a darker colour than the rest of the panel (sorry can't remember what it was, or even if it was one of the several available in the repo). Most of the themes don't seem to make the workspaces appear distinct.