I've shared my WiFi with my neighbour. How can I ensure they're not downloading illegal content?

I want to ensure they're not downloading any illegal content (torrent websites in particular). If they're using a VPN or Tor, then I guess there's not much I can do, but if they're just connecting directly to, say, pirate bay, without masking their traffic and downloading content through them, is there a way for me to spot that and tell them off?

I'm not interested in throttling their traffic or monitoring what websites they otherwise visit. I'm only interested in detecting if they access the popular torrent websites (pirate bay, kickass, demonoid etc.) and download stuff trough them.