I promised myself and I promised you I wouldn't fixate on Inception, BUT I went to bed last night pretty sure the whole thing . . . . from start to finish . . . took place in a dream.
And why? Because in the "real world" I could hear the ticking of a watch as part of the score, and this reminded me of a mysterious "tap tap" that occurs occasionally in Finnegans Wake that some theorize is a branch tapping on the window of the bedroom of whoever is dreaming the dreams that is Finnegans Wake. So this watch-ticking, I'm not saying that it is something heard by Leo through his dreams, but it is the first thing that made me say "Oh, this part is a dream" during "real-world" scenes.
The second, like I said in a comment, was when Leo squeezed between those buildings and then the Japanese Business Man rolled up in his car out of nowhere to rescue him.