Answer
You would have to do a series of experiments where the field changes, requiring the animal to remember the general location of the food source and use that to find their way in each new arrangement.
Work Step by Step
Some pieces would need to remain constant in order to make sure the metric being tested can truly be correlated with the formation of a mental map. Part of the challenge involved in such an experiment is making sure that $only$ the desired trait is being tested, something which is very difficult to deduce. Animals can use a wide variety of methods to find their way, so designing an experiment that requires them to use only the type of mental processing that is being tested can be difficult.
