cut him in half and count the rings?

Did you try asking it?

count its grey hairs

by looking at their teeth.

I am pretty sure that vets look at their teeth and know what to look for to determine approximate age.

ask him/her but it is rude to ask a lady her age!

If you don't already have a good idea, it is hard to tell exactly. And different breeds age differently. Smaller breeds age slower then larger breeds. So a small 8 or 9 year old dog may still look very young. While a large breed of the same age, will look and act very old. Many large breeds only live to be 10 or 11. Small breeds on the other hand can live any where from 15 to 20 years even. You can try looking at the teeth. If it still has thin sharp little milk teeth, it is still a pup. If it has it's adult teeth and they are still very clean looking it is probably a young adult. As the dog ages the teeth begin to look more discolored and worn. Older dogs begin to get gray hair round the face. If it is a large breed, it can start to gray as early as 5 years old. If a smaller breed it may not start to gray until it is 9 or 10. Just like people the older the dog gets, the grayer it gets in the face. If you have a dog with a really gray face he is pretty old. Older dogs get a grayish film on the eyes too. It can start off light and gets darker as they age. So white face and filmy eyes is a dog that probably only has a couple of years left to live. Like with people stressful living can cause a dog to gray out in the face faster.If you have a dog that was negleted or abused it may look and act older then it really is. Like wise a well cared for dog may age slower too. It can get confusing so take the dog to your vet and he can help you figure out the age.

usually a veterinarian can look at things like tooth presence/wear and give a pretty good guess.

can it be determined to the crumpled skin on their paws? ..