How to Check Customer is Subscribed to Newsletter by id or email Magento 2?

You can verify the customer is subscribed or not by the customer email id or Customer Id from the Newsletter Module.

Use Factory,
Magento\Newsletter\Model\SubscriberFactory

With isSubscribed() method you can check the Specific Customer is Subscriber to the newsletter.

The Result will be true or false based on the subscription status.

Output:
Boolean (True/false)

You can know the customer subscription status by email or customer id in the simplest way.