How can I know that my customer is happy or not without asking him this question?

Thanks

In my opinion ,

• When he-she come back to you to buy the same product.
• When he-she buying the product.
• When he -she sending people to you to buy your product.

You can see the Customer Reaction and facial expression once he gets the service/product, or if you analyse the list of customers who deals with the service/product if they keep getting it or not.

1. Their body language when they leave
2. If they visit you again
3. When they recommend others to buy from you

Happy customers are return customers, meaning that they will purchase again and again from your company. Super happy customers are those that come back bringing others with them. 

your customers feedback comes from your sales and profit ratio