Ask HN: How can I tell where current, live, activity hotspots in my city are?

I am in Chicago but this question could apply to any big city. It seems like a simple enough problem, but I can't find any method of answering the basic question, "where are all the people right now?" I tried looking at Foursquare trending, Yelp checkins, live camera feeds, and nothing seems to show me the basic information of how many people are hanging out, outside or in bars or restaurants, of a given neighborhood.

Open to ideas and suggestions. Thanks in advance.