curl --request GET \
  --url https://api.hubapi.com/email/public/v1/subscriptions/{email_address}

{
  "subscribed": false,
  "markedAsSpam": false,
  "portalId": 62515,
  "bounced": false,
  "email": "jerry@example.org",
  "subscriptionStatuses": [],
  "status": "unsubscribed"
}