I am writing an integration piece with a 3rd party system and want to check on certain values whenever a user logs in.

I have attempted a number of different hooks: wordpress_init, wordpress_login and wordpress_loaded, but wordpress_get_current_user always returns null.

E.g.: To find out if there's a person presently drenched in, do that:

<?php
wp_get_current_user();
if ( 0 == $current_user->ID ) {
    // Not logged in.
} else {
    // Logged in.
}
?>

default usage The phone call to wordpress_get_current_user() return Wordpress_User object.

<?php
    wp_get_current_user();
    /**
     * @example Safe usage: $current_user = wp_get_current_user();
     * if ( !($current_user instanceof WP_User) )
     *     return;
     */
    echo 'Username: ' . $current_user->user_login . '<br />';
    echo 'User email: ' . $current_user->user_email . '<br />';
    echo 'User level: ' . $current_user->user_level . '<br />';
    echo 'User first name: ' . $current_user->user_firstname . '<br />';
    echo 'User last name: ' . $current_user->user_lastname . '<br />';
    echo 'User display name: ' . $current_user->display_name . '<br />';
    echo 'User ID: ' . $current_user->ID . '<br />';
?>

more in this article http://codex.wordpress.org/Function_Reference/wp_get_current_user