It really is tough to make that call - I had to do it last month. I knew it was coming and had a couple of false alarms where the next day she was just fine. Then she woke up one morning and couldn't really walk, would take a couple steps and just lay back down and was just super lethargic. Called a friend who has spent a lot of time with her to come over and say goodbye, and I was apologizing because it may be another false alarm. She told me not to worry, to take her to Zilker and let her enjoy the park one last time that night, feed her hamburger and eggs and whatever is the best food she likes, and if she was fine in the morning, so be it. She wasn't fine in the morning, and so off to the vet one last time. It's tough as hell to do, but 16 (what mine was) is a long life for a dog, and I know I gave her an incredible 5 years (the time I had her), so at least my brain knows I did my best, even if my heart struggled with it. Good luck man.