Well, I don't believe you can unless you know it's birthday...
The only thing I can thing of that you could do is, take it to the vet...(If it's really important to know the age)...because if it's not that important, then I wouldn't take it to the vet, and have to pay a good bit...just to find out how old your dog is.
I am not sure if vets do this, but you can ask your local vet. They should be able to do a bone age test, or some kind of test to estimate the age of your dog.
all I can think of is either getting someone familiar with the breed to give you an estimate based on the dogs appearance, or have a vet take x-rays to look at bone ossification