I do not usually look for swarm cells unless I am in the hive to do a split. I do it the way you do it but I have seen several beeks, lift up the brood box and look under it. Swarm cells are normally on the bottom of frames.
Most of the time by the time you see swarm cells, it is too late to stop them from swarming or they have already swarmed. If you find swarm cells in the spring, try to find the old queen and move her. That way the hive thinks they swarmed and they may stay put.
Jim