https://www.envisionecommerce.com/blog/how-to-check-if-customer-is-logged-in-or-not-magento-2/
<?php
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$customerSession = $objectManager->get('Magento\Customer\Model\Session');
if($customerSession->isLoggedIn()) : ?>
<a href="<?php echo $block->getBaseUrl(); ?>/customer/account/logout/"> | Logout</a>
<?php endif; ?>