As Jay pointed out, if you find swarms cells, it it usually too late. Even if they are not capped, they are in swarm mode and it will be hard to stop them.
Best thing to do is not to wait for signs of swarming, but be ahead of them and do your best to prevent. Make sure they have plenty of room, especially in the brood area. You can also reduce the tendency by seperating the brood from the field bees with a double screened board (Snellgrove method).