Most broadband connections work really well with VOIP now but any jitter or packet loss will ruin your call quality.
Here’s a neat tool that checks your link for latency, packet loss and jitter. Bear in mind that if you’re running the test on a computer/laptop that’s connected via WiFi you are likely to get much worse results that a physically cabled connection. Running VOIP over WiFi is definitely not recommended.
The test requires Java installed on the machine running the test.
The test is provided by Visualware and can be accessed here – http://myspeed.visualware.com/indexvoip.php