I swear I once watched an episode of C.S.I. where the condemning evidence was the little yellow dots that color printers print on every page that allows users to be tracked. Even before that episode though, I felt that the information was common knowledge. The EFF has more details on the printers that do this as part of their Machine Identification Code Technology Project.
This video provides a simple synopsis on how to see the tracking dots and more information in general.
In addition, they’ve compiled a list of which printers have been observed using the yellow dots for tracking or other purposes.
If you’d like to take action against the mostly unpublished use of these tracking dots, you can find more information at SeeingYellow.com.