Magento2 - How to see if configurable products has simple products attached

Try below script from magento2 root:

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);

use Magento\Framework\App\Bootstrap;

try{

    require __DIR__ . '/app/bootstrap.php'; 

    $bootstrap = Bootstrap::create(BP, $_SERVER);

    $obj = $bootstrap->getObjectManager();
    $resource = $obj->get('Magento\Framework\App\ResourceConnection');
    $connection = $resource->getConnection();

    $sql = "SELECT * FROM catalog_product_entity WHERE type_id = 'configurable' and entity_id NOT IN (SELECT parent_id FROM catalog_product_relation)";
    $result = $connection->fetchAll($sql);

    if (!count($result)) {
        $sql = "SELECT * FROM catalog_product_entity WHERE type_id = 'configurable' and entity_id NOT IN (SELECT parent_id FROM catalog_product_super_link)";
        $result = $connection->fetchAll($sql);
    }

    echo "Number of products: " . count($result) . "<br/>";
    echo '<pre>';print_r($result);

} catch (Exception $e) {
    echo $e->getMessage();
}

If you are comfortable with using MySQL to answer your question, you could write a query such as

select * from catalog_product_entity left join catalog_product_relation on (parent_id = entity_id) where type_id = "configurable" group by parent_id having (count(*) = 0)

This will join the table for products with the table that houses the parent/child relationship. Then filtering that down via a having clause to find all the ones without that relationship.