Can't just replace 2nd, it rides on 5th also if i remember right, so you need 2nd drive/driven, and 5th drive/driven. SOmeone will corect me if I'm wrong on that?? Look for worn drive dogs, they will be obvious on the 2nd gear, if others look like it they will have to be replaced also. Measure the shift forks for proper thickness, and them being straight. The book will tell you pretty much what to look for, and usually the bad parts will jump right out at you.
I think I am alright as far as bad drive/driven gears I can see one dog that is rounded through the case and I will be able to tell if the shift forks are bad as well. What about a shift drum??? Or something like that. To my knowledge the gears and the forks should be it.