Magento – How to Check if the SKU is Exist?

mageissku.php

<?php
$sku = 'Your Product Sku';
$id = Mage::getModel('catalog/product')->getIdBySku($sku);
if ($id){
    echo "SKU {$sku} exists";
}
else{
    echo "SKU {$sku} does not exist";
}
?>

In 3 options above code will be useful

When you import magento products via code then this code is very useful.
If you want to load product via sku then this is very useful.
if you want to update any particular product via code then also this is useful.