Mark,
That part depends on you. There are so many ways to implement that function I decided not to make it a part of mohqueue. mohq_retrieve () provides a way to move calls out, mohq_count () allows you to check for queued calls within Kamailio scripts, and the database tables allow you to query it externally.
If you want to send information to the agents that a call is queued, you could write code in the Kamailio script to send this information when mohq_process () returns a true value. You could also externally query the database tables.
Hope this helps,
Bob