Well, seeing as how Obito went through with it, it's safe to say he wanted it the whole time, and possibly even instructed zetsu to tell obito. Zetsu probably informed madara of the war status as we saw this chapter. And the fact that there are no uchiha on the battlefield is a pretty good give away.
I would assume thru black zetsu cause he has been in contact with him this entire time. Also thru sage mode you can sense the world around you n he noticed all of them dead.