You have to select persons from your target group to be interviewed. And also your first questions in the questionnaire have the same purpose.
The target group for the questionnaire is not always representative to your market target group. If the packaging design is critical for opening and/or using in ergonomics the critical users are either disabled or elderly persons. In such a test you must focus on choosing test persons in the critical area.
If you are using mail questionnaires you have to find 100-200 addresses in order to receive about 50 answers. You can get such addresses from different sources. In the case with disabled and elderly persons you may contact associations to find the addresses.
Personal interviews can be done as a truly hall test. You just place yourself in a shopping mall. Select only to contact persons in your target groups E.I. children, elderly, etc.