It's quite difficult to keep people out of farms. I typically wait to see how they are shaping up toward the middle of the game. I like to see at least three or four cities (or the potential for them) before I commit. It's always a bummer to use up followers at the beginning of the game on a farm that doesn't pan out and then have to pass up a cloister because of it. I will always fight for the big farm as much as I can and try to get in the fringe undetected if possible. I also like to snipe the little ones at the end with my last few tile placements.