Cacher is the code snippet organizer for pro developers

<?php
/**
 * Check if there are onsale products.
 *
 * @access public
 * @param -
 * @return boolean
 */
function woo_have_onsale_products() {
  
	global $woocommerce;

	// Get products on sale
	$product_ids_on_sale = array_filter( wc_get_product_ids_on_sale() ); // woocommerce_get_product_ids_on_sale() if WC < 2.1

	if( !empty( $product_ids_on_sale ) ) {
		return true;
	} else {
		return false;
	}
	
}

// Example:
if( woo_have_onsale_products() ) {
	echo 'have onsale products';
} else {
	echo 'no onsale product';
}