Are you familiar with AMK and their catalog? It's available on-line.
What I do when I need to determine a bolt/nut/fastener finish is first locate the bolt PN in the AIM; The AMK catalog has an appendix that lists the GM part number and lists the page number in their catalog where the part can be found. Thereafter, they have a code that lists the finish. On page 6 of their catalog, they list the finish codes. Seems complicated, but I have this to be pretty efficient and accurate.