I don't know if it's still the case, but decades ago when I was at ISEF (and somewhat fewer years ago when I judged...) the judges would get to talk to the students.
I judged at some robotics competitions where we interviewed the students. It was usually immediately apparent if the student had not done the work, and as a result they would not receive an award.
I think that's the only way to do it. The student must be able to describe the work they did. A fraudster might still get through with sufficient coaching.
