Hi Stu,
You are right. The versioning is a bit more complex with SSRS and I took the path of least resistance in my answer ;-)
I was ignoring RS 2000 altogether.
I've looked at server properties and the ReportServer DB but there is nothing.
When you use the Reporting Services Configuration tool that shows a version number against Database Setup. Mine shows C.0.8.54.
I have a suspicion this may come from a config file in the web apps but I'm not sure.
Also when I googled for this, this thread was always on the first page returned. So it would appear it's not been looked for much.
I also think that if you are trying to do a universal check, that 2000 would be completely different to 2005, plus you've 2008 just around the corner.
With regard to different versions of SSRS to SQL, this is possible, but the SSRS upgrades should only affect the web apps/services and the ReportServer DB itself and the ReportServer DLL's for .Net perhaps. I'd be surprised if they affected the SQLEngine in anyway.
Also, you can design reports in BIDS in a different SP version than the server they are being deployed too, and that can create a few interesting issues!
I would surmise that something in the XML of a report definition might tell you what version it was designed in. I would also suggest a SQL trace to see if you can catch if the version number in the Reporting Services Configuration Tool/Database Setup node, is coming from SQL.
Not a great deal of help, I know!