The only way is in-person student code review or in-class projects. You could go over their code with each student and ask poignant questions that can only be a

The only way is in-person student code review or in-class projects. You could go over their code with each student and ask poignant questions that can only be answered if the student wrote the code themselves. But I'm not sure how feasible that or the in-class project is for most teachers. Also, anyone remember the story of the programmer who subcontracted his work to some offshore freelancer for a fraction of his salary? He just sat at his cubicle goofing off all day while the freelancer did all the work.