How can I determine my customer's core need?

I think even if You are a seller or part of a marketing team you must be focus on what others are doing (similar or better than You/your company). For me to have a conversation with costumers is a major point to determine what are their needs. But we must pay attention in details or signs that something is wrong. For example absence of shopping in your company/store; costumer doesn't talk openly with you, avoiding some questions. We need to be in contant alert for these things and call for a coffee or a tea maybe can help.