To test if cellular slime mold cells are attracted to cAMP, place their cells on a culture plate and place a drop of a solution of cAMP in the center of the plate or at some other location--attracted cells should always aggregate towards the solution. This plate should lack any food source for the cellular slime mold. As a follow-up, use a plate with a food source--if the cells always respond to cAMP, the result will be unchanged in the presence of food.
Work Step by Step
To get to this answer, see the text for information about how starving cells produce cAMP as a signal. It is necessary for the first experiment to have starvation occurring, thus, the absence of a food source. Then, the follow-up simply adds back the food source and looks for the same result.