Fluid loss of 5% of the body weight is considered mild dehydration, 10% is moderate, & 15% or more is severe dehydration.
One sign of dehydration is dark urine. This might seem strange but it's a good idea to be aware. Also if you aren't urinating as often as normal... this is another indicator. Failure to urinate for 8 hours or having extremely dark or amber colored urine are signs of severe dehydration. Drink your water.
How are you feeling? If you're experiencing headaches, lightheadedness, and dizziness, this could be another indicator of being dehydrated. Not drinking enough water will cause you to feel weak & tired.
While it is a late sign of dehydration, skin turgor is another way to check for evidence of being dehydrated. Skin with normal turgor goes back quickly to its original position while skin with decreased turgor remains elevated & returns slowly to its normal position.
Did you know if you're thirsty, you're probably already dehydrated. If your mouth is sticky & dry, you are dehydrated.
While this doesn't cover every clue to dehydration, these are some easy ways to check. To be on the safe side, DRINK YOUR WATER! Make it a goal to drink at least half of your body weight in ounces of water: If you weigh 140 pounds, please try to drink no less than 70 ounces of water each day. If you are exercising very intensely or spending time outdoors in extreme heat, you will want to be sure to consume extra water to allow for the moisture your body loses from sweat etc.
http://www.livestrong.com/article/252621-how-to-tell-if-you-are-dehydrated/#ixzz2Le6Nwd3q
http://www.nlm.nih.gov/medlineplus/ency/article/003281.htm