How to check Chevy 4L80E trans fluid? Hot , Cold , Running? In gear ?

Hi

I am trying to figure out the correct way to check the Auto Trans fluid in my 2005 Chevy Express van , 

Its has a Chevy 4L80E trans, 

Google says all kinds of things , from cold to hot and running
.....

What does the hive say ?

And is there a special fluid for the 4L80E ? , I have no idea whats in it now , do the fluids mix and match ?

Thanks for your help.

In reply to californiamilleghia :

Yes, it does matter what fluid you put in there.

Here's a link to the GM Owners manual.  I would follow the procedure and recommended fluid type listed.    

See page 6-13

Hot and in park.  DEXTRON-III H-Specification

You can download your owner's manual here -> link to PDF and find the answers those questions and many others yourself.