How does a dog act when it is getting ready to die ...

Dogs, like people, are not all the same.

Some dogs might choose to go away and experience their demise alone. People who are romantic tend to believe this is because the dog doesn't want her owner to suffer, but I'm not sure about this theory.

I had a dog who was very old, and by the end of her years couldn't really walk or even stand, and she just died in her sleep one day. 

Before my dog died a few years ago, he looked very very pale and he had the most touching stare. He also went into a quiet corner away from everyone. :(

That's sad

The loss of a dog is just like the loss of someone in your family

My "winnie dog" pased away from a heart attack.   She was laying on the coach,brought her little head up, "screemed" and was gone.  Poor little thing;to this day, I miss her.

Sometimes we need to stop suffering ... and  let our dogs go peacefully. Get a good vet that will do a three step process sedation, anestitize, then the last shot. It is very peaceful and so the right thing for dogs- but so very hard for the humans...

"... If you're sexually active, there's a chance you can still get pregnant, so you should use a backup form of contracep- tion ..."

The Cleveland Clinic Guide to Menopause