How to see a repeat customer on Shopify POS?

[PlanetRibs]

We sell food products online and via Food truck.
We use Shopify POS.

We want to reward repeat customers with swag. 

Since the initial purchase might be made a while ago or online - how can we flag it to the Food Truck operator?

I do not see ppl posting their credentials on POS at a food truck - all they do is the contactless payment.

So we need to track the repeat customers by the credit card, what options from Shopify back end or add-ons do we have?

Thank you, Aleh