Dehydration symptoms?

I know that lots of people are slightly dehydrated whether they had WLS or not.  I know that being dehydrated can cause you to be tired, lightheaded, feel hungry, have a headache, be nauseated, etc.  However, what are the symptoms that actually send people to the ER for fluids?  How do you know you are SO dehydrated that you need medical help?

Everyone is probably different, but just just telling me your experience would be interesting and informative.

Thanks,