With my 6 years of nutrition and health experience I feel overqualified to give 10 common and uncommon signs of dehydration that might just make you question your current health.
Your urine is yellow to brown
This is the most common symptom people recommend you look at to check your current state of hydration. Contrary to popular belief though, it is not a sure sign! You could have darker urine from fasting and although your body is in relative balance (balanced electrolytes is really what I consider proper hydration of the human body), since you're detoxing the toxins or sediment coming out are darkening your urine; combined with a lack of excess water your body needs to get rid of too.
Also rarely mentioned in mainstream health circles is that pure water is dehydrating. Just drinking the recommended 8 glasses could heavily dilute your urine. If you currently don't own a TDS Meter to test the "Total Dissolved Solids" of the water you drink, get one.
By drinking water with near 0 ppm or even up to 10 or 20, you may want to test your urine too (trust this, urine is sterile, nothing to worry about germ-wise here) and if it's higher than the water, that water is leeching minerals out of your body! Now sometimes that may be good, sometimes that may be bad. It may be good if you're body has excess sodium in it, which is incredibly common among Westerners for our high salt (from processed and cooked foods) diet. But if you believe you're in pretty pristine health such as you're vegan, raw, or watch your sodium intake like a hawk, you could be losing another mineral that is more important. At that stage it could be best to reduce your water intake. This alone will make your urine darker but I believe just the same amount of sediment will come out because it's mostly what the body doesn't need.
A book I read Eat for Heat by Matt Stone helped convince me the current "stay hydrated, drink 8 glasses a day!" mantra was bogus, though I disagree with much of what else this book talks about like adding salt to your food. Sodium Chloride is an artificial compound and is dehydrating. It needs water to balance it out, in fact for every 100mg of sodium taken up in your body needs 8oz (1 cup) of water to balance this out! And the USDA is recommending 2,000mg of sodium daily! That would mean 20 cups of water to balance you out, not that lame 8 cups! - So I try to eliminate all added salt whatsoever. Luckily even some canned foods these days, like tomato sauce, have a "no salt" version you can indulge in, otherwise they're packed with salt:
This is why your body retains water weight when you have a high sodium diet, and loses it when you go off a high sodium diet. Because that sodium would hurt you without the buffer! 400mg of sodium in your body results in a 2lb increase in weight. I think only 50 to 200mg of sodium a day are needed. This is what you'd get with natural whole foods. A far cry from the RDA but even other countries have lower amounts than 2,000 and did you know they recommend a 1,500mg limit for African Americans? Hmm, didn't know food was racist right? Race specific dietary guidelines from government...
The roof of your mouth is rippled, not smooth and flat
This is an unconventional fact that I again didn't discover until recent years. All my life I thought those ripples on the roof of my mouth were normal. Weird feeling, but normal. Then bang; I go vegetarian and reduce my processed foods, salt intake and gluten, and increase the fresh fruit, the roof of my mouth becomes smooth and I feel better. Even signs such as improved eyesight with less strain in the day are had when I'm properly hydrated.
See this mouth here, this person is so dehydrated she developed bumpbs; and I know this is likely the cause because I've had the same thing happen to me multiple times after eating heavy foods that are dry, and not balancing with water or hydrating foods properly:
Notice the ripples. And it is hard to say the least to balance out improper nutrition like a sodium overdose type of meal you get at Olive Garden. I'd almost bet you could get all your daily recommended sodium (which is already 20x too high) from an appetizer.
You're hungry...or thirsty, I guess
Confused? Let me explain. A lot of people mistaken hunger for thirst. First of all we eat way too much over the dietary guideliens. And keep in mind these guidelines were written by the food industry! They were already likely overinflated; just remember the food pyramid scam?
Men on average are supposed to eat 2,300 calories and women 1,800 calories. Well, sit down for this one, the calorie consumption in reality is over 50% more at an average caloric intake of 3,770 for Americans!
You are sweating
Yes, you sweating quite a bit may be a sign of how out of balance (and dehydrated) your body and it's electrolytes are. For over 20 years of my life I didn't know what a balanced body felt like. Just a few years ago I noticed a shift in my sweating under stress; that is, it was nearly gone or heavily reduced! It also did not smell bad or taste salty. If this had come on out of the blue I would have thought "wow, Mind Blown! But I was preparing for changes in my body after going strict on healthy foods for as long as I could (whole foods and to my dismay I cannot seem to stray strict for pretty long, but keep whole foods as a good 50% of my diet).
You smell bad
This ties in to the last sign. When you have too much sodium (usually from junk food) in your diet, for one reason or another, this causes a big rise in body odor. I have a few theories for this, such as the sodium drying (and thus killing) your skin and then a certain kind of bacteria flourish eating that dead skin (much under your arms obviously) OR the sodium is excreted in the sweat glands, again, under your arms usually, that kills the skin's natural bacteria and it smells from that dead bacteria either decaying or byproducts of gasses from other bacteria eating up the dead stuff.
Hashtag 'It's Complicated' could be the answer or maybe a combination of my two very similar theories. Test it out. Eliminate all your added sodium (it's in pretty much ALL processed foods, don't kid yourself) for maybe 5 days and see if you smell less. From my experience, it takes at least 3 days bare minimum to re-balance your body when your electrolytes are noticeably out of balance, i.e. you're dehydrated
One of the more common and sometimes obvious signs of dehydration IS simply dry skin. But, like I was years ago, I assume a lot of people don't even know what it's like to have hydrated skin without some external lotion or chemical applied. Hell, the water you shower in with chloride and it's derivatives will dry your skin and you won't know what it's like to have naturally hydrated skin if you can't go a few days without a shower.
It may take drastically changing your diet and reverting back and for for you to notice what it's first like to have soft, smooth hydrated skin and then watch disappointingly as it dehydrated from that tasty restaurant meal you had last night.
You're Stressed Out
This one is simple. Dehydration means a currently bad state of health for you. And bad health brings the opposite of happiness. So on the traverse, being hydrated brings health and happiness.
Hydration = Health = Happiness = Less Stress
Shortness of Breath
I saw this personally, and tragically, with my own mother. I could tell via her intake of food vs water day to day and the onset of shortness of breath or frantic breathing that dehydration was very linked. And then I looked it up and I was totally right.
More so a sign of being out of balance, because headaches can come on either end of altering your diet. For me it more so came when coming off of a high sodium binge than actually when getting onto one. Also watch for low blood pressure when reducing your sodium, and altered sleep patterns, which is why it sucks trying to balance tasty junk food with super healthy whole foods. Just remember, slow transitions are better on the body, it will adapt.
You have a sickness
In conclusion, I think a lot of diseases actually stem from or are aggravated by dehydration. There's even a book of a doctor who cured hundreds of patients of all kinds of sickness just by having them drink more water (I admit, the water also helps energize the person and flush out toxins as well as hydrate them).
[You're Not Sick, You're Thirsty](Water: For Health, for Healing, for Life: You're Not Sick, You're Thirsty!) is the name of the book, by Iranian F. Batmanghelidj - his other similarly titled book was Your Body's Many Cries for Water