It would not be easy to determine this, but we could do some things to try to isolate reasons. Our tests would depend on what we think is causing the decline.
First, we could take polls. We could ask people if they spend more of their retail dollars at these kinds of stores, whether they spend less, or whether their spending has stayed the same. If they spend less, we could ask them questions designed to determine what factors have caused them to take their business elsewhere.
Second, we could look at the characteristics of the kinds of stores that are taking business away from the kinds of stores you are asking about. We could look to see, for example, if Walmart-type stores are growing. We could look to see if more specialized shops are growing. When we find types of stores that are growing, we should compared them to the department stores and supermarkets to try to see what characteristics they have that the department stores and supermarkets lack.