Five rules to follow if you're certain you've caught someone cheating
1-Take the situation seriously.
Plagiarism is a serious accusation to make. Think of the consequences of a false accusation.
2-Gather your evidence.
Confirm you have enough to substantiate your claim
3-Determine who has to know.
If you're an instructor, you are likely bound by an official policy and should follow the procedures defined therein. If you're a parent - the issue is between you and your child.
4-Meet formally with the student.
Present your case and (barring a successfully mounted defense by the student) explain the consequences to them (there must be consequences).
5-Evaluate your role.
Plagiarized.com considers prevention to be the most important element in the fight against plagiarism. Did you follow the prevention guidelines?