There are basically two ways to monitor the disk queue, at a high-level from the Web UI or at a low-level from the individual node statistics.
From the Web UI, click on Monitor Data Buckets and select the particular bucket that you want to monitor. Click "Configure View" in the top right corner and select the "Disk Write Queue" statistic. Closing this window will show that there is a new mini-graph. This graph is showing the Disk Write Queue for all nodes in the cluster. To get a deeper view into this statistic, you can monitor each node individually using the 'stats' output (see Section 6.2, “Viewing Server Nodes” for more information about gathering node-level stats). There are two statistics to watch here:
ep_queue_size (where new mutations are placed) flusher_todo (the queue of items currently being written to disk)
See The Dispatcher for more information about monitoring what the disk subsystem is doing at any given time.