Tap, tap, tap. If you can't bust the fool using Internet techniques, check dates of books and articles cited in footnotes of a suspect paper. If all citations are five or more years old, before Internet use was popular in academe, chances are the plagiarist bought the paper from a cynical TA still paying off gambling debts. In the end, students have become more brazen because too many professors lack the time or the will to nail a suspected plagiarist. That affects academic standards.
Plagiarism is more than theft.
It represents a challenge to your integrity and expertise and puts your reputation on the line. Go online and preserve it.