Two ways: First we can scan for correlations, and see the
list of values which can be correlated. From this we can
pick a value to be correlated. Secondly, we can record two
scripts and compare them. We can look up the difference file
to see for the values which needed to be correlated.  In my
project, there was a unique id developed for each customer,
it was nothing but Insurance Number, it was generated
automatically and it was sequential and this value was
unique. I had to correlate this value, in order to avoid
errors while running my script. I did using scan for
correlation.

You can read the following detailed description on how to
correlate LoadRunner script:
http://motevich.blogspot.com/2007/10/loadrunner-scripts-correlation-tutorial.html

It shows what is correlation in LoadRunner and how to
perform it.

You can read the following detailed description on how to
correlate LoadRunner script:
<a
href="http://motevich.blogspot.com/2007/10/loadrunner-scripts-correlation-tutorial.html">correlation
in LoadRunner</a>

It shows what is correlation in LoadRunner and how to
perform it.

what is correlation? it is nothing but capturing the dynamic
data(an unique value)generated by the server.

if you find that unique value in your script, you could
correlate it easily. how you will identify those unique
values? you have to go through the script, if you come
across some values which has more no. of digits, please
concentrate on that whether it is unique. it might occur any
no.of times in the script. this doesn't mean it is not
unique. while recording the transactions you could guess for
which actions id will be generated (any dynamic values) and
based on that transaction another sub transaction could
occur so this sub trans will carry the same id generated for
the trans. thus unique id will occur any no.of times in the
script.

i like to share one simple example from my projects:
agent creating report for his client, when report is created
successfully,report no is generated. it is an unique id for
the report created. this id will be seen in the script after
your end transaction of create report. you have to check it
in server response of the script. if the id is there then
you have correlate this. thus while recording itself you
should have some idea where an unique id will be generated
for what.

if there is any controversy in this description please let
me know, so that i ll correct myself.