we introduced that capability in UM 9.9. In Enterprise Manager, you can click on a channel (linked to an IS doc type or JMS topic), then go to Named Objects and see the number of outstanding messages for that trigger.
The same information is also accessible using the UM Admin API.
In UM 9.12 we made it even easier, with visibility in Command Central. In Command Central there is now a single table view of all durable subscribers where you can see the number of outstanding messages for all triggers in one place. You can even sort the table to bring any stuck triggers to the top.
This information can also be accessed using the Command Central CLI or REST API.
For earlier versions, the only way to figure out the backlog is indirectly by looking at the last Event ID on the durable subscriber and comparing it with the last event ID on the channel. The bigger the gap, the bigger the backlog.
Hope this helps.