We'll be relying on a lot of different things.
But the truth is there is no way to 100% confirm it. No matter what measures we put in place there is always a way around it (and if people know all the ways we check they will just know better how to avoid it). The best thing to do is just join in and do your best. The best entries will be more of a reflection of the skills of the designers than the time they spent on it in my opinion.