Install the firebug from here
Press F12, In firebug window click on
element selector (i.e. arrow on firebug
option tab) Now click on message text,
you will see a div element which have this "
data- sigil='message-text' " attribute.
This div have data attribute which is named
as data- store and you are done.
This data have a property
"author":100001xxxxxxxxx
where this value is the author id mean page
admin id. (i.e. 100001xxxxxxxxx)
Note: If That Page Managed By Many admins
it shown the writer of that post (not main
administrator) Try In Your Luck