Conditional tag to determine if the user is viewing the Add Media (after clicking Add media button)

There are multiple actions available. Demo:

add_action( 'upload_ui_over_quota', 'wpse_78085_callback' );
add_action( 'pre-upload-ui', 'wpse_78085_callback' );
add_action( 'pre-plupload-upload-ui', 'wpse_78085_callback' );
add_action( 'post-plupload-upload-ui', 'wpse_78085_callback' );
add_action( 'post-upload-ui', 'wpse_78085_callback' );


function wpse_78085_callback()
{
    # see wp-includes/media-template.php
    print '<pre>' . current_filter() . '</pre>';
}

Result: