Fire up System Profiler... If the version of the ROM is:
$77D.40F2 -> Rev 1
$77D.45F1 -> Rev 2
$77D.45F2 -> Rev 3
Or... look at the ATI chip on the logic board. If it says "ATI Rage II+', then the logic board is Rev 1, if it says "ATI RagePro" then it's a version 2 logic board.
Hope that Helps...
John