I've talked about Brix Networks in the blog here before, but they've got a nice tool to check from any desktop whether the QoS is good enough for IP video. It can be found at: http://www.testyouripvideo.com/

It downloads a small Java applet into your browser to do a speed test, and then gives a nice detailed breakdown of the stats. From my home network (ADSL from British Telecom) I get a round-trip latency of 200ms and packet loss of 3%; Not ideal but still quite useable.