When dashboards are added to layout templates, there is currently no way to find this association unless you use the "Login As" feature and visually scan each tab of each layout template and make note of the dashboards.
From Workfront: "Currently, we do not have a report that would be able to provide you with what Dashboards are Associated to which Layout Templates. The best way to verify what Dashboards are currently active on a Layout Template would be to login as a user that is assigned to the layout template. "
This also cannot be reported upon, as Workfront stated above. We should be able to report on this, and we certainly should be able to see which dashboards have been added to a layout template after they are added.
The below layout template has a dashboard already added to this tab, but nothing is visible on the tab when choosing the layout template under the Setup menu: