Use Factory,
MagentoNewsletterModelSubscriberFactory

Using isSubscribed() method. You can check the user is a subscriber to the newsletter or not.

<?php
namespace MagemonkeySubscriberModel;

use MagentoFrameworkExceptionLocalizedException;
use MagentoNewsletterModelSubscriberFactory;

class IsSubscribed
{
    /**
     * @var SubscriberFactory
     */
    private $subscriberFactory;

    /**
     * @param SubscriberFactory $subscriberFactory
     */
    public function __construct(
        SubscriberFactory $subscriberFactory
    ) {
        $this->subscriberFactory = $subscriberFactory;
    }

    /**
     * @param int $customerId
     * @return bool
     */
    public function isCustomerSubscribeById($customerId) {
        $status = $this->subscriberFactory->create()->loadByCustomerId((int)$customerId)->isSubscribed();

        return (bool)$status;
    }

    /**
     * @param string $email
     * @return bool
     */
    public function isCustomerSubscribeByEmail($email) {
        $status = $this->subscriberFactory->create()->loadByEmail($email)->isSubscribed();

        return (bool)$status;
    }
}