Observe: This is a passive way to find out who are your customers. You observe and see what are some characteristics in common among your customers. What are they buying? How much they spent on your business? And so on.
Ask: Ask and you shall receive. Ask them in cashier, or gently talk to your customer in store and ask this question: Which ad bring you to our store today? Why are you here? What do you like most/least?
By doing this you will be able to understand your customer better.