Any idea why they do not test after each game, a couple of players from each team? In many competitions, including cycling, that's when athletes are tested, not before the game/competition. In cycling, the stage winner and the GC leader are tested in every stage, plus a handful more of riders. You could equally pick the player that scored the most points for each team and a couple more, randomly.
Having done my graduate degree at a university where football was big, it was noticeable how players became notoriously more muscular during the summer. I doubt there is any testing there.