In order to find queen cells either brush the bees off the comb with the
traditional goose wing or with a handful of grass.  Only shake if it doesn't
matter if the potential queen is damaged.
 
Never destroy a queen cell unless you are certain the colony can replace it if
necessary.  I have often had to help beekeepers who have destroyed queen cells
a week after the colony has swarmed.  They have only themselves to blame that
the colony is queenless.
 
You do not know whether the colony is queenless unless you  have challenged
them with the classic test of a frame of larvae.  I have often found that the
new queen will not begin to lay until all the previous queen's daughters have
emerged.
 
Chris Slade