He doesn't need to see the top fall over. He knows it's real. It's why he couldn't stay in limbo with Mal any longer, it's why he could let go of her and his guilt, it's why seeing his kids in limbo would never be enough for him. Dreaming is no substitute...not for Cobb.
I'm not sure how Cobb "knowing" it's reality is any kind of proof that it's reality. Mal was very convinced that it wasn't reality, because she could look at her own children and see they weren't real. Why would you trust Cobb's instincts over Mal's? Just because he's the main character?
He knows he's in reality because he remembers how he got to the house. That's what the whole long trip through the airport and meeting up with Miles is about. He doesn't just find himself at his in-laws' house; he "remembers how he got there".
Yeah, but plenty of stuff in that final sequence could indicate that those scenes were a dream. How the heck did Miles end up in the US before Cobb? Did Miles even mention he was going to meet him there? And wasn't Michael Caine wearing the exact same clothes in the airport that he was wearing in his earlier scenes?
How did Cobb get through customs so easily? He was a fugitive, wanted for killing his wife. Certainly his face would be all over the 24 hour news channels. He breezed right through customs just because a powerful CEO made one phone call? Even if Ken Watanabe worked his influence to get all charges dropped against him, it's very hard to explain why there wasn't a whole media circus waiting for him at the airport.
I get the feeling that a lot of people think the ending was "real" because they really want it to be real. Not because the evidence supports it, but because they think it would make the whole movie "pointless". But I find that a pretty silly line of thought. Was The Wizard of Oz "pointless" just because it was all Dorothy's dream?