how to get the size of message queue from akka actor?

There are Akka (Java API) actors (UntypeActor) and messages in the application. According to API contract in case a few messages are addressed to the same actor, they are queued and processed with one-by-one.

I'd like to handle messages in the actor depending on queue size. Basically: is there at least one more message queued at the moment current one is handled in Actor.onReceive()? What is the Akka way to archive this?