I actually answered a similar question over on Pets not long ago... Warm weather walkies and water... however you've asked in a bit more detail so here goes...
How can I know that the dog is perfectly hydrated?
The dog will refuse water given to them if they feel like they've had enough to drink - this is a slim balance especially in male dogs, as it's a fine line between him wanting to mark (lots of short pees) and therefore needing more to drink. But if your dog refuses water on a hike, they're probably not thirsty yet... but keep offering them water, especially when they start to pant more (this isn't a clear definitive reason to give water as they pant from excitement too!). Also if it's your own dog and they have wet food they can be less likely to actually drink due to the high percentage of water in their daily meals. Male dogs are not like humans in regards to peeing and hydration because of marking, they will force a pee at any point, and try and retain water for this reason.
How much water is enough?
Until they stop drinking. Big dogs will drink more water, small dogs less - but different breeds will deal with heat better than others... huskies probably want more water in the heat than say a short haired tan Labrador. I always take my Shetland Sheepdog his own flask of water - either he drinks from a bowl or the cup from the flask, and it's HIS flask, so I can pour the water back in if he doesn't drink it all, and if there's mud and bits in it's fine (dogs prefer bits in their water to the stuff straight out of the tap!) - That said, if you know you're going to be by say a river or stream that's got a good flow - don't take anything, just let them drink as they go.
How can I know that it needs water?
This is a little more tricky as dogs pant for several reasons, but if it's really very hot, and they're panting, offer them water. They may also whine but not all dogs will do this for water. If they nuzzle empty bowls and try and lick slightly damp surfaces then offer them a drink! I think as well here it's better to let them have a settle in some shade too where possible. Offering the dog you were with water with every break is really the right way to go. A dog can't really say 'Yes please I'd like some water' but if they see you drinking, and they pant and whine they're probably thirsty! They can get dry noses if they're really thirsty but they can get this from any number of other reasons - like if they've wiped their face, or there is a high wind.
It would drink a lot of water every-time I offered. (I just poured water into my palm and it would drink)
This actually may not have been 100% thirst - you may have had deliciously sweaty palms. Every time I work out or go for a run my dog loves to lick my palms as they're sweaty, and the salty taste is just irresistible.