If it still has the FW it was shipped with, you could tell by that.
Look for the FW version (like XXKE2 for example) and the salescode/CSC that is currently installed (*#272*<your-phones-IMEI># - VIA for example).
Then look in the FW download threads in the dev section, with some some luck you find a FW with the same combination, usually there's also info from where it is.
(XXKE2 and VIA from the above examples would be German MultiCSC, o2 (VIA = ViagInterkom = o2) to be more precise - it's what I had).
Worth a shot.