7.2. What Constitutes Academic Misconduct
If you are not sure what constitutes cheating, discuss this with the instructor and other TAs. Certain activities (such as collaboration on assignments) may or may not be permissible in the class you are TAing. It is your responsibility to know what is normative in general (e.g., all TAs should know what the University of Minnesota considers plagiarism), and the instructor's responsibility to clarify any grey areas or special rules. Two websites that might be useful are the Office for Student Conduct and Academic Integrity site, which contains a couple of FAQs as well as links to university documents like the Students Conduct Code, and Academic Conduct Information for New CS&E Students.
If you notice suspicious activity and are not sure if it is cheating or not, gather any relevant evidence and discuss it with the instructor if you think there's a likelihood that it is.