Take a big scarf or towel and wrap it around the helmet of the goaltender. Cover only the front of the eyes and make sure to allow plenty of room for air.
When they look down, they should be able to see about 2-3 feet in front of their skates.
If they can, tell them to stand in a ready stance (stick on the ice, gloves out), then go about 5-10 feet in front of them and shoot slow, on-the-ice shots to their stick.
The idea is for goaltenders to react quickly to the puck and make the save. While vision is impaired, the 2-3 feet of visibility means the goaltender should be at least making an attempt to save or see the puck.