How do you test the effectiveness of preservatives?

The easiest way to test the effectiveness of a particular
preservative would be to add this item to its intended food product and wait.  The
preservative would be considered effective if it kept the food from spoiling.  Of
course, most companies would now also conduct bacterial contamination tests.  Government
agencies, like the FDA, help regulate the definition of what constitutes spoiled and
safe food products.  In order for the test to be valid, it must be tightly controlled. 
Items containing the preservative would have to be closely monitored and regulated. 
Each sample would need to be subjected to the exact same conditions in order to be
compared with a sample that does not contain the preservative.  While testing consumer
grade preservatives can be a lengthy process, the concept is as a simple as following
the scientific method.