It is unfortunate when it happens, but you may have to confront the possibility that a student has plagiarized or cheated on an assignment or examination. The University has carefully specified policies and procedures for addressing academic misconduct, and it is important for you to know what they are if you find yourself facing these problems.
The first step is to try to prevent these problems from occurring in the first place. Careful explanation of the definition and consequences of plagiarism, coupled with strategies for helping students get through the stages of writing a paper, will pay off later in better work from your students and an atmosphere of trust and consistency. Similarly, taking steps at an exam to ensure the secrecy of the exam and honesty during the exam, as well as reducing the temptation to cheat, will result in fewer student problems later on.
For more information on preventing or responding to academic misconduct, see these UW resources: