WP_Model-array-one-liner.php

<?php

function isAssoc($array){
	return count(array_filter(array_keys($array), 'is_string')) > 0;
}

$indexed = [
	'apple',
	'pear',
	'grape'
];

$assoc = [
	'fruit_1' => 'apple',
	'fruit_2' => 'pear',
	'fruit_3' => 'grape'
];

isAssoc($indexed); // false
isAssoc($assoc);   // true