Function to check if a WooCommerce order contains a free trial (via the Subscriptions extension)

order_has_trial.php

<?php

/**
 * Loop through an order's items and see if there's a free trial set
 *
 * @param WC_Order $order
 * @return bool|WP_Error
 */
function custom_order_has_trial( $order ) {
	if ( ! $order instanceof WC_Order )
		return new WP_Error( 'invalid_order', 'This order is invalid.' );

	// Grab all items in an order
	if ( $items = $order->get_items() ) {

		// Loop through the items
		foreach( $items as $item_id => $item_values ) {

			// Verify the product information is everything we expect
			if ( is_array( $item_values['item_meta'] ) && $product_id = $item_values['item_meta']['_product_id'][0] ) {

				// Check each product for a trial length
				if ( class_exists( 'WC_Subscriptions_Product' ) && WC_Subscriptions_Product::get_trial_length( $product_id ) > 0 ) {

					// Return early if a product has a trial length
					return true;
				}
			}
		}

		// Otherwise, we return false
		return false;
	}
}

Thanks very much for this, well written and works as expected!