We should put that on a FAQ.
Paged Messages are paged before they enter the queue. It's like caching the producer in a file at the server, and only processing the messages when the queue is consumed.
You can choose by either block or page.
There is a feature request to provide a counter at the address level.
And there's another feature request to page messages to disk at the queue level when dealing with slow consumers. (like subscriptions on topics).
At the moment you can get the NumberOfPages at the Address Level through management. Each page has a configured size.. so you can have an idea of how many messages are sitting on paging case the server is in page mode.