Feeling exhausted and really run down. Often your urine will be very yellow almost glowing. If you have been outside for hours without a drink. If you get a pounding headache that makes your head dizzy. If your month feels really yucky and dry and your lips have no moisture and are cracking you could be dehydrated. Try to have water or a drink like hydralyte.
If you have been vomiting that can often make you dehydrated too.