If you're lucky, the old GM was like me. I actually took old copies of RM2 and the companions we used, split them up, and put what I used in a three ring binder. I also typed up all my house rules in a standard rules format and had a binder with those, too. Finally there was the binder of revised Professions (we redid skills and skill costs almost completely and also heavily modified a few professions).
If not, Hurin's checklist is a great starting point IF the official companion rules are being used. If not, you might have to index on your own. A good general start is to group the house rules based on main RM2 book (ChL, AL, and so on) and go from there.