If the swarm cell is capped it could be to late. If they are just being formed with larva, royal jelly inside, and the bees are still elongating the queen cell it is usually not to late. If you can check every 10 days you should be alright with catching them at start swarm impulse but it is best if you can prevent them from getting that far. Just giving them lots of room by adding supers does not always prevent swarming. Keeping room in the broodnest to prevent congestion is a better way to put it.
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