Checking if a post with certain meta value exists

One way would be to use get_posts function:

$posts_with_meta = get_posts( array(
    'posts_per_page' => 1, // we only want to check if any exists, so don't need to get all of them
    'meta_key' => '_sku',
    'meta_value' => '1',
    'fields' => 'ids', // we don't need it's content, etc.
) );

if ( count( $posts_with_meta ) ) {
    // they exist
}

Currently it will search for published posts. You can customize it to your needs, so you can search for different types of posts or for posts with different statuses.