What type of experiment could you do?

Maybe raise a colony of aphids with ants and another colony without them, under otherwise identical conditions, and see what happens. If the ants are parasitic, the aphid colony without them will do better (have more members, healthier members, etc.). If the ants and aphids are symbiotic, then the aphid colony with ants should do as well as or better than the other colony, depending on whether the dangers the ants protect the aphids from are included in the experiment.