In the past we solved that by the instructor discussing the material with the students; it would quickly become apparent who knew something and who didn't. Unfortunately classes have become too big for that.
That injects a ton of bias into your marking though. Under that method, shy students and students with poor verbal skills (for example, ESL) don't stand a chance. I have seen excellent work come out of students that you could barely talk about the material, but could produce excellent assignments (this is in a domain where plagiarism would be very easy to detect, so I'm very confident it's actually their work).
It's better for shy students or students with poor verbal skills to work on that in an academic environment than to be dropped into a work environment where those things are just as important without their education ever bothering to help them with it.
"Interpersonal skills are optional" is not a good side effect of our current educational system!