How can a copyright holder find out if I have copyrighted materials on my computer?

The most likely way is if the materials are shared on the internet, either through publication to a personal website or file-sharing service. Your Internet activities are not private. When you connect to a file-sharing network, you can be simultaneously downloading and distributing materials. Your IP address identifies you to anyone connected to the same tracker, and they can easily see the files being shared by that IP.Copyright holders search the Internet to determine whether copyrighted material is being illegally distributed. They often search with the same peer-to-peer software (e.g. BitTorrent) used by those who share files. In a sense, they act like any individual looking for a particular file, say a movie. When they find a file, however, they issue a copyright infringement notice to the network provider from which the file was transmitted.