Back before we started using single-story brood chambers, we sometimes had
trouble finding the queen.  We would slip a queen excluder between each of
the boxes the queen might be in, then come back 4-5 days later.  The box with
the eggs in it had the queen, and we could focus our search on the one box,
instead of looking through 2 or 3.
 
There is a technique that I have never used--I will try to get this
right....basically, you shake all the bees out of the existing hive, through
a queen excluder, and into a new box.  The queen (and drones) theoretically
gets prevented from entering the new hive by the queen excluder.
 
Take a queen excluder and an empty deep super.  Spray some non-stick cooking
spray on the side of the empty super (this keeps bees from being able to walk
up it).  Put the queen excluder over a hive body with some frames in it.  Put
the empty super on top of the queen excluder.  Open up the hive you want to
requeen, and shake ALL the bees off of each comb into the empty super.  If
you smoke them gently, that will encourage them to go through the excluder
into the new hive body.  Take the boxes from the existing hive, and carry
them away from the site.  Shake all the bees off the boxes onto the ground.
 Carry the empty super and queen excluder away from the hive site.  Put the
hive back together at the original site.  The hive is now queenless--if you
have been careful to make sure that all bees in the box were forced through
the excluder.  (I hope I got this description right.)
 
This technique sounded incredibly messy and time-consuming when I heard it.
 It's easier to simply find the queen.  However, it does mean that you can
requeen a colony whose queen you cannot find.
 
The more you look for queens, the easier it will get--practice every time you
open the hive, and sooner or later you'll be able to spot a queen in moments.
 (My husband's sure-fire technique when he's having trouble, is to say a
little prayer... it's amazing how much easier queens are to spot when you ask
God to point them out to you!)
 
Good luck.