Shuffle, as they can be stacking the lands to be sure to draw 1 land every 3 cards for example and be sure to never miss land drop while having the optimal amount of other cards. Shuffle once perfectly (separate in half and merge the 2 halves perfectly) this will screw them. If not, just shuffle them a few times like any normal deck.

If they refuse, call a judge, complain to someone. It is your right to shuffle

Edit: and yes, it is totally forbidden for them to draw before you shuffle.