Lets start with defusing some common myths:

"only males flair their beard"  FALSE
- both males and females will display their beards.  Sometimes for a mear stretching of it or as a sign of
dominance.  Beard displaying can mean many different things.

"only males are Dominant and females are always submissive"  FALSE
- Both males and females can be Dominant or submissive.

"only females show body language of submission i.e: waving"  FALSE
- Both males and females can show body language of submission such as waving & bowing.

"you can visually tell the sex of a bearded dragon at any age"  FALSE
- It is near impossible to *accurately* determine the sex of a bearded dragon that is under 6" long.  By
9" it should be
very easy to determine the sex.
sex·ing
1. To determine the sex of (an organism).
2. The property or quality by which
organisms are classified as female or
male on the basis of their reproductive
organs and functions.
There are a couple of ways to determine the sex of your bearded dragon.  The method most commonly done and is the safest is the tail flip method.  
The other one is probing which I do not do nor do I recommend.  Probing should not be performed unless by a licensed veterinarian... Even then I don't
really feel that it is necessary to invade them this way when a safer and a reliable method is available.

To sex your dragon you first need to make sure that your dragon is at
least 6" long.  Once that is determined then place your bearded dragon in the
palm of your hand facing them away from you with your thumb gently on their back. With your other hand lift their tail to almost a 90° angle.  Be careful
not to bend it back too far as you can seriously injure your bearded dragon.

When the tail is at almost a 90° angle focus your attention to their
cloaca (vent).  Just above the cloaca you may see two bumps on the sides.  These are
called the
Hemipenes bulges .  If you see these two bulges and an indent between them somewhat resembling an hour glass - congratulations you have
a male!

If you see one bump or bulge right above the
cloaca or no bulge or bump then congratulations you have a girl!

Sometimes it is hard to determined for the inexperienced eye, but keep practicing and comparing and you will begin to really notice the difference.  
I have put pictures of both a male and a female to show you the differences in this area.

For young babies you
may need to slightly twist the tail back and forth by rolling the tail between your thumb and index finger.  Please be sure you do
the twisting gently!

Other tip for trying to determine the sex of your Pogona Vitticeps is look at the tail - females have a sleeker and more tapered looking tail.  Males tend to
have a thicker tail at the base.
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<--- Hemipenes bulge
Hemipenes bulge   --->
<--- Hourglass indentation
between the hemipenes bulges
<--- Cloaca most commonly
referred to as the "vent".  The
cloaca contains the genitals,
urinary and intestines
Male
One bump or no bump --->
just above the cloaca.
Cloaca (vent).    --->
Female
Below are two pictures... One showing a male and the
other showing a female and the differences between
them.  Sexing a bearded dragon can be tricky and
frustrating for beginners but practice makes perfect!
Exclusive
Dragons
Exclusive
Dragons
Written by: Robin Moniz 2006