Using the real time reports is the way I've done it in the past ...
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There's also on the Windows releases an .exe called showlicenseusage.exe (possibly prog files/wfavvid/bin) which dumps out teh current license usage and who's using it - that includes Supervisor seats as well.
In 9.0(2) there is finally (i believe) a report to show peak usage on a historical basis. Pre that it's all realtime or nothing.