You only can see download data and other analytics about apps you own if you go to iTunesConnect.com. Apple does not discuss information about other apps. However, there are statistical analysis to give you an estimated guess, they take publicly available information such as number of comments and stars, number of players on GameCenter, ranking and featuring, external links etc. They gather that info and then apply statistical methods to come up with a pretty good estimate. For example AppAnnie does that.
