Final answer:
A skin swab test can be used to determine the causative agent of a suspected staphylococcal skin infection.
Step-by-step explanation:
The diagnostic test that could be used to determine the causative agent of a suspected staphylococcal skin infection is a skin swab. In this test, samples from the wound are taken and cultured. Under the microscope, gram-positive Staphylococcus species are identified by their cellular arrangements that form grapelike clusters, and when grown on blood agar, the colonies have a unique pigmentation ranging from opaque white to cream.