A quick and easy check for dehydration in any dog is to pinch up their neck skin, as much as you can grab ahold of and pull it up against gravity (use the top of the neck if they're standing , a side will work if they're laying down) No need to be mean, just lift that extra loose skin up and let go.
If it quickly slides back into place with no hesitation, no hydration needed.
If it lags going back into place and leaves a little hill of skin poking up where you grabbed, they need a drink, but probably not bad off.
If it barely slides back into place or leaves a big handfull sticking up, I'd go and get an IV or subq fluids right now.
This isn't just a blasto thing, If you have hard working dogs, or hike out during the summer it's good to know. My vet put in a bag of fluid and the associated tube and needle in my K9 first aid kit.
Another good one to have handy is one of those nearly instant read thermometers. Many dogs have different core temps, it's a good thing to know what your dog's base line temp is. Granted, it's not the most pleasant thing, but the dogs seem pt mind less than the humans do! (Also, try not to confuse the human thermometer, from the doggy one. I colored the doggy one with a big brown magic marker, so you can even tell in the dark and not put something in your mouth that you really don't want to)
Lastly, I have another doggy health "dip stick" reading. Pull up a lip (the dog's of course) and look at the gum above the big canine tooth. Should be pink and glossy. (or pink with spots depending on your dogs mouth colors) Press the pad of your thumb against the gum firmly for a few seconds and then release. Observe how the blood fills back in on the depressed tissue. Quickly? Does the less pink depressed gum fill back up and turn back to bright pink promptly or does it lag? Does the gum seem grayish?
Those are my 30 second check under the hood to tell if a dog is just acting weird or acting sick.
Anybody else got any?