Not sure if there is a mod for this. In base game you just have to watch the farm for a sec to see if there’s a vil walking around on it gathering. Only one vil can gather from each farm at a time. For what it’s worth, if you have farms sitting idle and a villager nearby finishes a farm, before they go to reseed they will first seek out an available farm to work on, so this problem more or less solves itself over time. But I understand why you’re asking, it can be tough to re-task your vils to all the idle farms if you accidentally double click your vils or if you repurpose a big group of farmers to build a castle or something like that.