The best way to figure it out is to identify the CE_ pins on the ROMs and then take an ohmmeter and figure out what they're connected to. Check the CE_ pins against all the pins in the cache/rom slot. Check the CE_ pins vs. GND and vs. +5/+3.3. Install a cache/ROM module and test the resistance again (no power necessary).
Simply trying it seeing what happens doesn't tell one enough. If it doesn't work, there could be many different causes. This will eliminate the possibility of one specific potential failure.