Check for queen cells
Queen cells are special cells in which new queens are reared. There are always some naturally empty queen cell cups in the comb, usually in the lower corners. If there is an egg in the queen cell cup, it is always an indication of swarming. In fact, finding these eggs, or even grown queen larvae, in cell cups is the most important sign of swarming behavior. Checking the colony frame by frame is is a lot of work, but it is also a sure-fire way to detect swarming. In most cases, carefully checking three frames from the middle of the brood area will reveal swarming behavior, if it is present. Tilting the brood hive body and inspecting the lower edges of the frames is a fast method for detecting swarming behavior and it also reveals most cases of swarming behavior. If swarming behavior is detected, such as eggs in queen cell cups, additional steps should be taken to handle the swarming colony.