Correct you can tell when they are going to swarm during a check of the hive and see queen cells which are along the bottom of the frames and larger than either drone or worker cells.
We tried an empty hive this spring and while they checked it out none of the swarms were interested in moving into it. So he tries to catch them right after they swarm from the hive since they usually only go a few feet away first until they decide which new home to head for. Plus, the queen can't fly very far at a time since she usually doesn't do any flying after her mating flights. We keep shrubs about 3 feet away from the hives for them to land on and we also have fence posts they sometimes land on.
