How to check if PHP array is associative or sequential

Question

Can someone please suggest to me an efficient way to check if an array "is a list"?

I want to be able to differentiate between this:

$sequentialArray = [
    'apple', 'orange', 'tomato', 'carrot'
];

And this:

$assocArray = [
    'fruit1' => 'apple',
    'fruit2' => 'orange',
    'veg1' => 'tomato',
    'veg2' => 'carrot'
];

Any help would be appreciated.