At the moment you can only see a plain listing of all existing workflows in the “Workflows” overview list. Unfortunately, it is not possible to see whether a particular workflow is a (or used as) sub-workflow and whether it is currently used by another parent workflow.
The idea now is to display an indicator (small icon etc.) in the workflow list that makes it clear that the workflow is a sub-workflow (e.g. if (among others) the Execute Workflow Trigger node is used in it).
Furthermore, it would be very good if it were shown whether the sub-workflow is used by other workflows (reference to the sub-workflow in another workflow in the Execute Workflow node). For example, by colouring the indicator.
Finally, it would be very helpful if, when deleting such a sub-workflow with dependencies to parent workflows (reference to the sub-workflow in another workflow in the Execute Workflow node), a warning with a list of parent workflows should pop up.