Determine if entity type is fieldable
There might be a simpler way but I think this should be pretty reliable:

// Inject the entity_type.manager service instead if possible.
$class = \Drupal::entityTypeManager()
    -&gt;getDefinition($entity_type_id)
    -&gt;getOriginalClass();

$interface = 'Drupal\Core\Entity\FieldableEntityInterface';

if (in_array($interface, class_implements($class))) {
    ...
}