I always liked the idea of tattoing a small S on the stomach next to a spayed dogs (soon to be scar). On the sole basis, if lost and adopted out, wouldnt have to be opened up again to see if was fixed or not. Plus all the hair covers it anyways and you hardly see it.
A lot of working dogs, are tatooed in the right ear. The Tatoo numbers are held at the registry. This is so you can prove the dog is from the litter you say it is. Though you can see where this can be faked if you have a dog of similar age, breed, and know the tatoo number.
I believe a dog's finest hour, the greatest fulfillment he holds dear, is when he has worked his heart out in a good cause and lies exhausted on the field of battle-with a ball in his mouth."