Distinguishing between new and returning users

How do I tailor my skill based on whether user is a first time user or not?

I ideally want to introduce the idea that there is a subscription version of the product on the 2nd or 3rd time the user returns to the skill. How do I figure out whether or not they are a returning user or not? Would I store the data in AirTable and call it back? Or is there another way?

Thank you so much! This is really helpful.