You could start by dumping the raw data that get_field gets for you and see if that matches your expectations in various scenerio’s (checked, unchecked, field not existing).
I use a snippet like this to dump things (because I don’t like how var_dump() looks):
echo "<pre style=\"text-align:left;padding:10px;border:1px solid red;background-color:#fff;color:#111;font-family:Courier;\">";
print_r( get_field( 'featured' ) );
echo "</pre>";