Magento – check if attribute value already exists

I'm not sure how .NETs SOAP API would handle this, but I'd try using the catalog_product.list method with a SKU filter. If the resulting list/array/collection/whatever-its-serialized-as is empty, then you know the product doesn't exist.

Per the comments below, the following code should get you that list

Dim productExists As Boolean 
Dim webService As MagentoService = New MagentoWS.MagentoService 
Dim filters As filters = New filters() 
filters.filter = {New associativeEntity() With {.key = "sku", .value = sku}} 

Dim productInfo As catalogProductEntity() = webService.catalogProductList(sSessionId, filters, String.Empty) 

productExists = productInfo.Count() > 0

Thanks @DavidManners for the hint. In the end, it was easier to look how it is done Mage_Catalog_Model_Resource_Abstract::_setAttributeValue though. I came up with the following script which checks if a product has the "Use Default Value" flag set for a specific attribute:

<?php

error_reporting(E_ALL);
ini_set('display_errors', '1');

require_once 'app/Mage.php';
Mage::app()->setCurrentStore(Mage_Core_Model_App::ADMIN_STORE_ID);

$store = 7;
$entityId = 15087;
$attributeCode = 'meta_title';

// easy with loading the whole product
/** @var Mage_Catalog_Model_Product $product */
$product = Mage::getModel('catalog/product')->setStoreId($store)->load($entityId);
echo 'Has Product a Store View Specific Value? ';
var_dump($product->getExistsStoreValueFlag($attributeCode));

// tricky to do it without loading the whole product
$adapter = Mage::getSingleton('core/resource')->getConnection('core_read');
$attributeModel = Mage::getModel('eav/entity_attribute')->loadByCode(Mage_Catalog_Model_Product::ENTITY, $attributeCode);
$table = $attributeModel->getBackend()->getTable();
$entityTypeIdField = Mage::getModel('eav/entity')->setType(Mage_Catalog_Model_Product::ENTITY)->getEntityIdField();
$select = $adapter->select()
    ->from($table, array())
    ->where($entityTypeIdField . ' =?', $entityId)
    ->where('attribute_id =?', $attributeModel->getId())
    ->where('store_id =?', $store)
    ->columns(
        '*'
    );
$values = $adapter->fetchAll($select);
echo 'Has Product a Store View Specific Value? ';
if (empty($values)) {
    var_dump(false);
} else {
    var_dump(true);
}

I did not test it, but the second version should be much faster as the product does not have to be fully loaded.