Tried Scopia once with Avaya on the phone for 3 hours trying to connect to a LifeSize setup in our office and it was a nightmare.
Anyway, just google VTC test sites, as long as the desktop client supports H.323 video conferencing you should be good. This is one of my favorite test sites to use since it works almost all of the time -
http://vtctest.pointsofdata.com/
Also, Polycom has a list of test sites somewhere if you can find it and some of them play a demonstration. I've had the best success testing with their site in Israel and the worst was testing with any of the US sites.