An invention has commercial potential if it can produce or be incorporated into a product that a customer will buy at a price that produces a profit. In a very real sense, the ultimate customers of any invention are people like you and me. They have families, homes and jobs. They are born, they pay taxes and they die. Because potential customers are real people, in some instances, you actually can ask them whether they need the invention - however, it is recommended that you not ask your Mom, because she will love anything you have invented. In those cases in which the invention is totally new, you may have to show them a drawing, working model or prototype of the invention (preferably under an agreement of confidentiality) before they can understand that they need it.
In that inventions do not produce or become products by themselves, customers are also those who decide whether to license inventions or whether to invest in their development. Again, you can ask - and preferably did ask before you started inventing.
Consider who the (potential) customer is and which need(s) the invention would fill. Is it a new need or an old one? Is the need associated with a long-term trend (e.g., the aging of the Boomers) or is it a short-term need?