As a seller or marketer, how do I know that my customer has a need at all or not?

As a seller or marketer, how do I know that my customer has a need at all or not?

One should identify certain cues or stimuli that create need for a product. We often experience during our visits to the local store, when the shopkeeper asks us what we are looking for. That is the prime question to identify the basic need of the consumer. In another case, as in a costmetic shop for eg., the salesperson would observe the customer's facial skin which may be having acne or freckles and then he would immediately identify the need of that customer and recommend him to buy a particular brand of anti-acne cream. Basically the observation of behaviour, body language, attitude and psychology of the consumers are essential in order to identify his need or drive.