what variable can i use to identify a specific pages
You may have trouble adding CSS with drupal_add_css in hook_preprocess_page because the $styles variable has already been built.

Better to add CSS in hook_preprocess_html or hook_page_build

Note, you should be able to target specific pages in CSS by using body classes.

e.g:

body.page-node-type-article #content{
  background-color:pink;
}


But yes, to check the path, you can use 

$arg = arg($index); // system path component
$path = current_path(); // full base system path ($_GET['q']);
$alias = $_SERVER['REQUEST_URI']; // full alias only