While this may not seem like a large number in relation to the size of the university, it is a serious issue.
Plagiarism can blatant or subtle.
The following list of questions is meant to help with detecting plagiarized papers, particularly those which are wholly or largely copies from other sources. There will probably be more than one indicator that a paper has been plagiarized in whole or in part. Your own instincts as an instructor are also important. Online paper mills often include old papers with even older citations. Most undergraduate students do not interlibrary loan much or any of their cited works.