I found the following QoS tester tool that seems to work very well for stress testing routers or servers to find bottlenecks in your network:  http://www.vconsole.com/client/?page=page&id=23

It allows you to test jitter, packet loss, and out of order packets over a specified UDP port between two networks.  From that data you can use an online mean opinion score (MOS) calculator to figure out the estimated voice quality.