For WCF hosted on IIS you can use ASP.NET performance counters to see number of queued requests, current requests etc. for example:
- ASP.NET\Requests Queued
- Web Service\Current Connections
Although WCF has a lot of performance counters available, there is none indicating queues length.
If you are talking about queued messages content, there is no easy way. The only I can think of is taking a full memory dump of the process and investigating it. But this is of course very intrusive, single shot task.