You cannot tell if water is drinkable just by looking at it, you might be able to tell if it is not drinkable, but that’s totally different. Even “natural spring water” is no longer considered safe until it has been chemically tested or cleansed. There are many sources of water that can be used for emergency water purification; including groundwater, reservoirs, rain, or seawater that of course would have to be distilled to be drinkable. You would need activated charcoal, bleach, or iodine in order to make these water sources drinkable without fear of getting sick. So make sure that your emergency kit contains these things so that you will be able to take care of your family in an emergency.