Add a new icon, perhaps between the worker and farm icons, to notify the arrival of wagons (both regular and trade). Don't show the icon if no wagons have been sent to avoid cluttering the interface unnecessarily.
Currently, you have to remember this or set yourself alarms. This is very impractical when you send wagons several times a day and every 2-3h you have to remember to set it again.
A red flashing icon would be a much better reminder of this.