The discoloration of leader is probably a relatively good method, but might depend on variables other than simply age, namely light & heat. But, of course these things play just as important of a role as time does. I have some very old (no idea to be exact) TMAX 400 in bulk rolls, and I can say that the leader is significantly lighter than the inside film.

Package design is probably going to be the best bet, and it would be cool to get a database of that.