way to test web filter appliance for content filtering weakness?

Hello,

We install these expensive web filters (websense, sonicwall cfs, etc) and
then how do we test its effectiveness in certain categories?

Does anyone know of a utility or website that might do a test on web
filters? We pay a lot for them and have no way but brute testing (which is
worthless and endlessly time consuming, not to mention we're NOT interested
in seeing the websites we need to test!)

If anyone has any ideas, utlities or anything, I'd be glad to hear it!

Thanks,
Matt