how will I know when its time to put my dog down ? He is a 15 year old coon-hound lab x depressed scarred of the wind n rain sleeps so much has a hard time getting up his back hips are going drinks a lot of water a day and when I mean a lot I mean a lot hides in the bush etc..

While that is entirely your choice, I feel the need to ask has he been to the vet to address these problems? Excessive drinking could be caused by many things, just like humans there are many physical changes that occur in aging dogs also and any change should be addressed at the time of the change.
Arthritis can be managed with meds.
That being said at 15, he is officially geriatric and this decision should be addressed with his vet on the chance that he can be managed and kept comfortable.
Personally speaking I could never euthanize one of my pets without knowing first that there were nothing I could do for him to ease his discomforts and pains.
Humane euthanasia may be the best choice for him, but only you can make that decision.