Good morning, 

I recently presented to a group of ten local police chiefs on the topic of Tor and its more common, though less publicized, civil usage, as opposed to the more publicized criminal usage. During my presentation I emphasized this fact, and they countered that because its usage is entirely anonymous, it is reasonable to assume that it could be used entirely by criminals and none of the oppressed as I had claimed. 

Which brings me to my question. Other than private individuals saying "Hey, I use Tor for X", how does the overall network know what the network is used for, even approximately?