WP_Check_Image_Size.php

/**
 * Function to check weather or not an image sizes exists in the thumbnail library for a specific attachment id
 * 
 * @param $attachment_id - the id of any image in the se
 * @param $image_size_name - the name from add_image_size()
 *
 * @return bool
 */
function check_image_size( $attachment_id, $image_size_name ){

	$image_info = wp_get_attachment_metadata( $attachment_id );

	$sizes = $image_info['sizes'];

	if( array_key_exists($image_size_name, $sizes) ){
		return true;
	} else {
		return false;
	}


}

/** Usage */
$image_exists = check_image_size( $image_id, 'thumbnail' );