How to detect if an ACF custom-field is really in use?

Most sensible here is probably just a custom query

global $wpdb;

$sql_find_meta = 
"SELECT post_id
FROM database_name_here.wp_postmeta
WHERE meta_key = 'my_special_value'
AND meta_value > '';"; // > '' == not null, whitespace or blank

$posts_with_meta_key = $wpdb->get_col( $sql_find_meta );

foreach ( $posts_with_meta_key as etc.....

You can repeat this for wp_termmeta and term_id

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