How to tell the age of a dog?

How can you tell the age of a dog? I work at a shelter and all of the vet techs look at their teeth but I never have asked what they are looking for. Are they just seeing how dirty the teeth are and then guesstimating?

Ask the vet techs.
They'll tell you.

When they look at there teeth they are looking at the size and the color and that is how they determine the dogs age!

if yellow is at the top they are 2 years old or more