Today I needed to run a quick check to see if a WooCommerce Sensei Lesson was complete. It took some digging, but the answer is:

$user_id = get_current_user_id();
$value = WooThemes_Sensei_Utils::user_completed_lesson ( $post->ID, $user_id );
if ( $value == false ) {
	//Lesson Not Completed
} else {
	//Lesson Completed
}		

Enjoy!
Toby