The first sign that you're getting dehydrated is dry mouth where your mouth begins to get dry so if you're experience dry mouth then you're beginning to get dehydrated and need to drink some water or something hydrating.
Some other symptoms of dehydration is lethargy and dizziness as well as a headache.
You may also experience vomiting, nausea, dry eyes, muscle weakness, muscle cramps, Lightheadedness and heart palpitations.
If you're just dehydrated a little bit then drinking some water can help with the dehydration but if the dehydration has gotten too severe then you need an IV through medical attention to recover from the severe dehydration.
If you think you or someone is severely dehydrated you should get to the emergency room or call an ambulance because the dehydration if too severe can lead to unresponsiveness and even death if not treated in time.
To keep yourself from getting dehydrated make sure you drink plenty of water and other hydrating fluids before you get too thirsty.
Especially during really hot weather you should continually drink water to stay hydrated and don't wait until you're becoming too thirsty or dehydrated.