Finding a queen can be very difficult, especially in a full hive. It's like playing Where's Waldo in real time! When I first became a beekeeper, I did not find the queen until my second year! Now, that is not a problem :) but even with years of experience she can sometimes still escape my eye!
There are only a few cases in which you actually need to find the queen or locate the queen.
> You are requeening
> Splitting
> Combining 2 hives
There is no secret formula to find her, but there are some tips that will increase your chances. Queens will generally be on frames of brood, usually uncapped, as that is where they are needed. You will usually not find her on the outermost frames of honey. She is longer than the rest of the bees and has a shiny thorax. It helps to slant the frame you are checking, often times the sun will glint off her body. Another trick is, when you pull out a frame, look down into the hive, often times you will see her, head down, running from the light! If you leave a hive open for too long, the bees will begin to move off of the brood and onto the walls of the hive. At that point it is almost useless looking for her.
It is best to inspect frames for the queen over your hive as sometimes they will fall off the frame. You don't want to lose that little lady in the grass!
If you just simply want to verify that you queen is in the hive, Check the brood nest for eggs. If there is freshly laid worker eggs, chances are good she is alive and well! It take 3 days for an egg to hatch. So she would have had to been in there at least 3 days ago! Eggs look like little grains of rice on the bottom of the cells. Try tilting the frame and let the sunlight illuminate the bottom of the cell!