ECM & EPROM cross reference sources?

My son bought a 92 cutlass with a 3.4L. There was a basket of "stuff" that can with it including 3 ECMs. Since all this was included I think someone may have changed or was thinking of changing the ECM. Is there a source on line to find GM's part numbers? I don't want to change anything just verify that the correct one is installed. I pulled the codes and have 12 which didn't come up on a list I found. That's what made me question what had been done. Any help would be appreciated.