One way you can find a queen bee in a hive that you can clearly see is to find an area where you see a large clump of bees all looking into one central area for a long period of time. Also, some bees will repeatedly come and go from this same place. In the middle of this clump, if you were to brush the workers away, you would most likely find the queen. She is normally guarded by as many as 10-20 workers at one time.
Many times it is not necessary to locate the queen herself as there is likely a lot of evidence of her presence in the hive. Capped brood is the most compelling evidence that you've got a viable queen. Look for a uniform deposition of eggs (if it's spotty it may be a laying worker or an old queen).
If I missed something in here, or if you have more info to add, please do edit this article.
Brood is capped 9 days after the egg being laid. The best sign of 'queen rightness' are eggs (second to seeing the queen herself). If you can find eggs they were laid in the last 3 days. A laying worker will generally stick eggs to the side, rather than bottom, of a cell as her abdomen is not long enough. Workers can only lay drones (male) as they will not be fertilised. You can alos get a drone laying queen however, if she is old, damaged or poorly mated.