Not quite sure what sort of response you were after as that is not an area they were concerned - bit like asking how police monitor end users of illegal substances were tracked.
But for a while in the mid 2000's the likes of RIAA and MPAA were suing grandmas and uploading fake and infected files to various websites and p2p and Usenet in an effort to discourage people from seeking such files
They don't monitor it specifically. Rights holders deploy honey pots, usually connected to P2P swarms which log the IP address of the downloaders that are connected. The rights holders then contact the ISP with the content, time and address of the offender and the ISP sends you a warning notice under that three strikes legislation.
RIANZ are the only organization taking advantage at the moment. Nobody has received a notice for downloading a movie or TV show to my knowledge. Popular music tracks are the ones that will get you pinged.
There is a legal grey area when monitoring infringing people this way. It has been claimed if the rights holders are allowing it to be downloaded for free from the source, then it is free game. I don't think that has ever been argued in court at all, let alone successfully though.
Sometimes what you don't get is a blessing in disguise!
A couple of years ago a meeting was held between NZFACT and the dvd rental industry. It was explained back then that no one would be issued an infringement as the movie companies can't be seen to accept to pay the $25 charge to pay the ISPs.
Other countries that tried to charge for issuing notices has been rejected by the movie studios. If they are seen to accept it here, then they'd have to accept it everywhere and end up costing them more in notice fees than they would lose in income.