How do I check on the size of the queue? I know it's not JMS standard, but most JMS implementations have a way to do this.

You can view the queue depth using the MBeans in ActiveMQ 5.x. Use any JMX management console to see the statistics. See How can I monitor ActiveMQ.

You can also browse the contents of a queue using the JMS QueueBrowser.

Or you can access statistics programmatically

Through the MBeans, you can monitor individual destinations including message statistics related to the destination. For example, you'll see the following attributes on a destination (Queue or Topic):

• Enqueue Count - the total number of messages sent to the queue since the last restart
• Dequeue Count - the total number of messages removed from the queue (ack'd by consumer) since last restart
• Inflight Count - the number of messages sent to a consumer session and have not received an ack
• Dispatch Count - the total number of messages sent to consumer sessions (Dequeue + Inflight)
• Expired Count - the number of messages that were not delivered because they were expired

The "size of a queue" is also explicitly called out in an attribute "QueueSize." QueueSize is the total number of messages in the queue/store that have not been ack'd by a consumer. This can become confusing at times when compared to the Enqueue Count because the Enqueue Count is a count over a period of time (since the last broker restart) while the Queue Size is not dependent on a period of time but instead on the actual number of messages in the store.