There's no surefire way to tell how old a dog is without knowing it's exact birthday but you can estimate the age of the dog.
A large breed dog will usually look like a puppy even at 24 months of age or more.
So if you have a puppy and it starts growing out of the puppy look then they're likely over 24 months of age.
A puppy will be at least one month old when they start growing their milk teeth. So if you see teeth starting to grow then they're at least a month old.
15 years in human years will equal one year in a dogs life.