There are several of how wasps communicate with each other, including chemical, mechanical and visual signals, as well as situational cues.
For example a forager wasp who returns with a load of water might have the water unloaded by a worker wasp right away. That tells the forager there is a high demand for water, and she might decide to get more water immediately. If the forager has to wait with her cargo for a minute or two, she might deduce that there is no pressing need for water, and not depart to forage right away, or to go and forage for something else.
If they find a good source of food or building materials they lay a pheromone trail so that other wasps from the colony can follow it.
If they are panicked they emit another pheromone signal to alert other wasps from the colony of the danger, and these wasps will often fly to the signal to see if they can help.
Wasps recognise each other by sense of small and by visual recognition, see later article.