The easiest ways I find for identifying a ROM 0/ROM 1 vs ROM 3 is just by looking at the motherboard:
1) ROM 0 & ROM 1 boards have the battery soldered onto the motherboard via leads, whereas the ROM 3 has a black plastic caddy that the battery clips into
2) ROM 0 & ROM 1 boards have headers or solder points for keyboard and numeric keypad, whereas these are gone on a ROM 3
3) ROM 0 & ROM 1 boards have "Apple IIgs" written horizontally on the board (looking from the front), whereas the ROM 3 has "Apple IIgs" written vertically behind the memory expansion slot
There are also the number of chips etc, but the methods above are the "easiest" to spot.
Hope that helps.