What are some signs that your dog may be passing on soon?

I have a German Shepherd/Norwegian Elkhound mix who is about 12 or 13. She's already outlived her life expectancy of around 10, according to what I've read. She's been uninterested in food, treats, and even people food for a few months. Some days are better than others. She has never been one to eat dry food without turning her nose up, so I usually mix in some wet. She doesn't really even want to eat that lately....she'll leave most of the dry food behind. She is pretty slow moving and not excited about much. She spends most of her time in her bed, or sleeping on the kitchen floor, and she's constantly drinking water. I don't feel like she's dehydrated...she isn't panting or anything like that. I'm thinking it's just her time...I would love to hear your thoughts, or experiences. Thanks.