Depending on the product, it may be customary to not only perform end-of-shelf-life studies, but to also perform studies at various intervals throughout the entire shelf life of the product. In this way you can establish expectations for product performance.
As has already been pointed out, these studies need to be planned (designed) as any experiment and documented with a baseline of the fresh product in order to have a point of reference for comparison. The usual or expected holding conditions need to be defined and recreated for the experiment, e. g., ambient room temperature, refrigerated, frozen, in darkness, humidity, etc.
For some products it is customary to perform what are called accelerated shelf life studies, in which the experiment design purposely includes anticipated abuse conditions, e. g., out of specification temperatures that the product may undergo during transport or in a retail display case.
Aside from the already mentioned microbiological and chemistry testing that would be appropriate for a potentially hazardous food, shelf life studies may also focus on organoleptic attribute testing for aroma, color, mouth feel, taste and array of visual changes. A chocolate chip cookie may never present a food safety issue during its shelf life, but it may undergo quality degradation, losing its crunchy ,chewy, moist, characteristics.
As may or may not be self evident, it can be dramatically less expensive to perform some preliminary shelf life testing in house as opposed to sending out to a certified lab. Depending on the equipment and space at your disposal, competencies of your staff, complexity of the desired experimental design and the labor hours available to you, many shelf life studies may be carried out in house.