I think most cheater are caught by the large amount of A`s these kids give themselves.
I always checked the grades I gave after they were in the database, and caught five of them changed: all D's changed to A's. Apparently this was done by a Department head known as "Blake the Flake", since I turned them into her and they were whited out and changed on the sheet.
I reported them as misrecorded and they were changed back, and I never heard anything more about it. I told other people in the Department and several more were found.