$post_check = new WP_Query( array(
    // http://codex.wordpress.org/Function_Reference/WP_Query#Custom_Field_Parameters
    'meta_query' => array(
        array(
            'key' => 'foobar',
            'value' => 1,
        ),
    ),
    'posts_per_page' => 1, // Just checking if it exists, we don't actually need the results
    'no_found_rows' => true, // We don't care how many total posts match these parameters
    'update_post_meta_cache' => false, // Don't pre-fetch all custom fields for the results
    'update_post_term_cache' => false, // Don't pre-fetch all taxonomies for the results
) );

if ( $post_check->have_posts() ) {
    echo 'A post exists with foobar=1';
}