When your students submit the project, have the landing page give them 10-15 minutes to make one tiny change and resubmit. Pick something that would be trivial to do in seconds for a student who did his own work, but would cause a lot of stress for someone who outsourced.

Save the 1:1 interviews for students whose original program was great but whose modification went down in flames and you'll likely be talking to your outsourcers, plus identifying students who maybe could use some extra help even if they did everything themselves.