This paper presents how to detect MP3 files that have been downloaded from peer-to-peer networks to a user hard disk. The technology can be used for forensics of copyright infringements related to peer-to-peer file sharing, and for copyright payment services. We selected 23 indicators, which show peer-to-peer history for a MP3 file. We developed software to record the indicator values. A group of selected examinees ran the software on their hard disks. We analyzed the experimental results, and evaluated the indicators. We found out that the performance of the indicators varies from user to user. We were able to find a few good indicators, for example related to the number of MP3 files in one directory.