re: How do you know when it is time to put down your dog?

If the dog isn't happy and is in constant pain, you should put him down. If the dog still loves being by your side and still has, "good moments" it's hard because I think maybe the dog thinks that all the pain is worth the few good moments they do get, however fleeting. It's a judgement call, but dogs are fiercely loyal animals and you should cherish all the time you have.