The Rev 4 or 6 should be silkscreened on the lower left hand corner of the motherboard. Further factory enhancements were usually indicated by the addition of a tiny white sticker somewhere near the top of the Zorro slots (but may have since fallen off). Rev 4 boards have iirc 32 chip ram chips while Rev 6 have only 8. You can usually tell an upgraded Rev. 4.x board up to Rev.4.5 iirc if you see a resistor soldered onto the top of the small chip located between the cpu slot and the first Zorro 2 slot (U605?). The uprated boards should also have a different Gary chip (MOS), a modification near the mouse connector and a resistor added near the power supply connector. There's a good description on how to upgrade a Rev. 4 boards to 4.5 on Usenet.
