Magento – Magento 2: How to check If custom options exists for product

I was working on something similar today - I would try to get all current options, push your new option(s) onto that array and do this:

Mage::getSingleton('catalog/product_option')->unsetOptions();
$product->setProductOptions($options);
$product->setCanSaveCustomOptions(true)
    ->setHasOptions(true);
$product->save();

The first line is explained here (not sure if you'd need it actually): https://stackoverflow.com/questions/4006260/magento-accumulating-custom-options-in-script

Edit: By the way, this code is working perfectly for me, however I'm using it in a script in development

I fallowed this way it is working may it will useful for you

<?php

require_once 'app/Mage.php';
umask(0);
Mage::app('admin');
$option = array(
    'title' => 'custom option title',
    'type' => 'drop_down', // could be drop_down ,checkbox , multiple
    'is_require' => 1,
    'sort_order' => 0,
    'values' => getOptions()
);
$collection = Mage::getModel('catalog/product')->getCollection();
foreach ($collection as $product_all) {
    $sku = $product_all['sku'];
    $product_id = Mage::getModel('catalog/product')->getIdBySku($sku);
    $product = Mage::getModel('catalog/product')->load($product_id);
    $product->setProductOptions(array($option));
    $product->setCanSaveCustomOptions(true);
    $product->save();
    echo $sku;
}

function getOptions() {
    return array(
        array(
            'title' => 'Ship It',
            'price' => 0,
            'price_type' => 'fixed',
            'sku' => $sku,
            'sort_order' => '0'
        ),
         array(
            'title' => 'Avon',
            'price' => -5.00,
            'price_type' => 'fixed',
            'sku' => $sku,
            'sort_order' => '1'
        ),
         array(
            'title' => 'Bristol',
            'price' => -3.00,
            'price_type' => 'fixed',
            'sku' => $sku,
            'sort_order' => '2'
        ),
         array(
            'title' => 'Barkhamstead',
            'price' => -3.00,
            'price_type' => 'fixed',
            'sku' => $sku,
            'sort_order' => '3'
        ),

    );
}