The obvious answer to me is in usability testing.
Define a series of tasks that you want to test the packaging for. The obvious task for packaging is…well, opening it. The next obvious task is about the content displayed/printed on the package. What is the product? What are the primary features that the packaging tells you about? Once you have the tasks, you test several sealed packages with participants. You record them as they try and open the package, as well as describe to you what they think of the package's printed content. You run them through the content-specific tasks you have defined. You have them do a standard usability questionaire after the tasks, one that is rewritten to align with the tasks tested.
You can even do A/B testing of different versions of the packaging if you have the time and capability.
After analyszing the test results and the questionaires, you should have a fairly good idea what works and what doesn't. As the opening of a package is one of the first "experiences" that most customers have with a new product, more companies should be focused on designing the packaging (and evaluating and measuring this) than actually are.