The easiest way to catch the girls before they all fly off on you is to tip up your boxes every week and look for queen cups that have larva in them. 95% of the time they will have the swarm cells on the bottom bars of you second or third box. Only once did I miss a swarm by not catching the cells on the bottom of the frames. That colony actually built all of them on the sides of the frames, i could not believe it.
The best thing you can do is to not let them run out of room but they will still swarm when they get to a certain population, new queen or not, some strains sooner than others.