It's not uncommon for someone to adopt a dog and not be sure how old he is. Once dogs are adults they are difficult to age. Factors that can help age a dog include:
Tooth development – Dogs get their adult teeth at known approximate ages and therefore can be used to help "age" a dog. However, by six months of age, dogs have all their adult teeth making it very difficult to "age" a mature dog.
Tartar development – The condition of the teeth can be helpful as dogs will develop tartar and calculus with time. However, the rate of tartar development varies from dog to dog and can be difficult to use.
Size – Evaluation of a dogs size to determine if they are full grown can help "age" a dog. This may be difficult in some dogs, especially mixed breed dogs, as you don't know what their true adult size is supposed to be.
Attitude – In some cases, this can be somewhat helpful in situations where a dog really "acts like a puppy". However, this is not very reliable as some dogs will act like "puppies" all their life or mature late. Some puppies may have a calm demeanor at an early age making them appear older.
Gray Hair – Some dogs will have graying of hair on the face, especially around the muzzle. It is not reliable at dogs can gray at different ages, just like people.
Eye changes – As a dog ages, the lens of the eye becomes more dense and makes the pupil look "cloudy". This change is a normal aging change called lenticular sclerosis. This change occurs most often around 5 – 6 years of age.
All of the factors above have limitations. Many times, aging an adult dog will be based on a combination of the all of the above factors combined with experience of seeing lots of dogs of which you know the ages.
If you want to know your dogs age, ask someone with lots of animal experience such as your veterinarian.