You can do it a few ways. But in my experience, the simplest and best way is to force people to demo something at a pre-specified time. If you're doing it "hackathon" style, then it should be at the end of the hackathon. If you're doing it 20% style, then probably end of the quarter? You could do it with zero accountability, but having tried to run these programs (see my other top-level reply here), I think you'll mostly get a lot of people not doing much if you don't have any accountability.