To test the quality or your VoIP connection you can use some web tools that give you some information

To test jitter and packet loss you can visit:
http://myspeed.visualware.com/indexvoip.php

It takes a useful test on the server around the world and codec that you choose. .

This is another webpage very similar where you can test your connection
http://www.voiptest.org/speedtest.swf

The results are useful to test your network and to find problems.