Well, once you open it up, your best bet would be to look for anything that looks burnt, deformed, or out of place. Sometimes, when circuits go, they leave a small burn mark on the metal or plastic of the enclosure that covers them. Step 12 of the guide is just a little note that generalizes what ifixit does, and sells, not uncommon since they made the guide. You find this in some of their older (2009) teardowns.