Comment by bluGill

Having driving through floods before: you don't. You either know the road before and thus know it is safe (though sometimes is washes out and you are wrong!), or you watch others and then follow the same path they did. In a very few cases there are signs that tell you (I know of a couple places where the road just crosses a stream, and the signs tell you when it is no longer safe)

You also need to know your vehicle. Some cars can wade through deeper water. Sometimes a heavy SUV will get through where a light jeep will float away. Other times the light jeep will get through and the heavy SUV gets stuck in mud.