From the Longhorn UI, go to the Volume tab. Each Longhorn volume is listed on the page. The Attached To column displays the name of the workload using the volume. If you click the workload name, you will be able to see more details, including the workload type, pod name, and status.

After the workload is no longer using the Longhorn volume, the volume detail page shows the historical status of the most recent workload that used the volume:

When a PVC is no longer bound to the volume, the following status is shown: