How is a field's key (rather than value) obtained?
You can change the display in your content type "Manage Display" and ask to have the key and not the label :)
It's here : /admin/structure/types/manage/[your type]/display

Programmatically for a nude value as asked :

$items = field_get_items('node', $node, 'field_list'); 
$key = $items[0]['value'];