Finding a queen can be one of the most frustrating tasks for a beekeeper. Even if the hive is not crowded, queens seem to make it their business to hide, and they usually do a good job of it.
That's why Slowmodem's answer is so good. If you can know the queen is there without seeing her, it is often (but not always) adequate.
Another way to know that the queen is present (even if there are no eggs) is to listen to the hive's "music" or read the bee's behavior.
A queen-right hive, when opened properly, will continue with its' business as if nothing has happened. A queenless hive, even if opened expertly, will respond to the "disturbance" with a distinct buzzing that soon fades away. The bee's behavior will be overactive and "nervous", seemingly running around the frames without a purpose.
Spotting either of these signs takes a bit of experience, but if you are aware of them and look for them, they are not too hard to recognize.
But, what about the cases when you MUST see the queen, like when you're catching a swarm or want to know if an old queen has been replaced? I'll jump to the "last resort method" and leave room for others to tell of their preferred methods.
If you force all the bees to go through a queen excluder, the queen will get stopped when she tries to go through. There are many ways to do this, I'll suggest only one here.
Place a super without any bees in it (but with frames) on the hive's floor . Above it, place a queen excluder and above that place the super where you think the queen is located. If there are more supers that might have the queen, do this to each, one at a time.
OKAY, now you use your supercharged smoker and fill the upper box with so much smoke that the bees move down. After the super is empty or almost so, raise the super and examine the excluder for the queen. If not there, check the almost beeless frames, one at a time for the presence of the queen. You should carefully remove frames 1 and 10 (those next to the walls), examine them, put them aside and make a space in the center of the box. Lift the frames one at a time, hold them by the sides and bump them down firmly on the super in the space you've made. Your hands not the top bar bump the top of the super and all bees get dislodged and thrown down onto the excluder. After all frames have been de-beed, check the excluder again for the queen.
If you've done this "last resort" method properly and haven't found the queen, than she isn't there.
But maybe she was and just flew off..... :'(
So now someone else will give you a more enticing, less extreme method, one that should be used before this suggestion.
