How do I find out how old my russian tortoise is?

I have a pet russian tortoise named Artemis. I bought her from PetSmart. I never did ask them how old she is. I got her about a year ago in December. She's pretty big! I'm guessing she's gotta be at least 5 years old, but I want to know the exact age she is. How do I find out? Please answer and help me out! Thanks so much!!!(: