Look at this picture:
http://www.news.com.au/heraldsun/story/0,21985,22556281-661,00.html

THE Right Brain vs Left Brain test ... do you see the dancer
turning clockwise or anti-clockwise?
If clockwise, then you use more of the right side of the brain and
vice versa.
Most of us would see the dancer turning anti-clockwise though you
can try to focus and change the direction; see if you can do
it.
LEFT BRAIN FUNCTIONS
uses logic
detail oriented
facts rule
words and language
present and past
math and science
can comprehend
knowing
acknowledges
order/pattern perception
knows object name
reality based
forms strategies
practical
safe
RIGHT BRAIN FUNCTIONS
uses feeling
"big picture" oriented
imagination rules
symbols and images
present and future
philosophy & religion
can "get it" (i.e. meaning)
believes
appreciates
spatial perception
knows object function
fantasy based
presents possibilities
impetuous
risk taking
Hint: look at the upper and lower part of the legs. They
are actually on the same 'level', though it appears that one is in
front of another. Furthermore, you will only perceive the legs
moving in a single direction, though it is equally 'valid' of both
directions. If you observe closely (with mindfulness) around the
leg enough, you'll be able to 'switch side' at will. Of course if
you are not mindful you will not see it.
Not only does this image tells you what kind of 'brain' are you,
but more importantly it tells you how the same thing can have two
different way of perception.
The two way of perception in Buddhism may be known as 'samsara' and
'nirvana', though they are based in the One Mind, and One Reality,
and are non-dual. The sage views with transcendental perception,
the ordinary views with samsaric perception, but they live in the
same world.
If we can constantly observe the 3 dharma seals in all phenomena
with mindfulness at all time, eventually it will click -- one will
see it, an intuitive quantum leap of perception to Emptiness of all
phenomena will happen.
Regarding how to observe the three characteristics:
5. The Three Characteristics
As I wrote previously that Nirvana is Samsara rightly seen,
and,
As Nagarjuna said, "That which, taken as causal or dependent, is
the process of being born and passing on, is, taken noncausally and
beyond all dependence, declared to be nirvana." And as David Loy
said so nicely in The Difference
between Samsara and Nirvana: There is only one reality -- this
world, right here -- but this world may be experienced in two
different ways. Samsara is the "relative" world as usually
experienced, in which "I" dualistically perceive "it" as a
collection of objects which interact causally in space and time.
Nirvana is the world as it is in itself, nondualistic in that it
incorporates both subject and object into a whole which, Madhyamika
insists, cannot be characterized (Chandrakirti: "Nirvana or Reality
is that which is absolved of all thought-construction" ), but which
Yogacara nevertheless sometimes calls "Mind" or "Buddhanature" and
so forth.
As for what are we to be aware? There is no fixed objects, even
though in meditation one's mindfulness may be concentrated in one's
breathing. But it is not a fixed object but a whole
conditioned-arising-luminous-empty-process, and mindfulness does
not require any particular objects. We can be mindful all the
time.
When we are mindful, we will be aware of our thoughts, and not be
lost in our thinking. Necessary thoughts can arise but we are
conscious of them. Not only are we mindful in our thinking, we are
also attentive of everything -- even a speck dust that passes by,
one who is mindful and alert will be able to know it. For a Buddha
-- a flower that blossoms in another universe, he will know it as
if it is in the palm of his hands.
And the nature of all phenomena will be witnessed moment to
moment.
All these are already elaborated previously.