Work from this:
DECLARE @service_name sysname = 'SalesReturnService'
SELECT ce.*
FROM sys.conversation_endpoints ce
JOIN sys.services s ON ce.service_id = s.service_id
WHERE s.name = @service_name
AND ce.far_service = @service_name
AND ce.dialog_timer > sysutcdatetime()
You may have to fine-tune a little. I'm uncertain what is in the column dialog_timer when there is no conversation set up. (It is not NULL, if the documentation is to be believed.)
To remove the timer, use END CONVERSATION with the conversation_handle column from the query above.