Tests for infection. In order to treat an infection in a wound, a clinician must correctly diagnose an infection. A culture should be taken of any wound that may be infected before empirical antibiotics are administered. There are proper ways to perform a culture: for example, using the levine technique. This insures a more accurate method of determining an infection.
Visual. A wound infection is generally diagnosed just by looking at it. There will be redness, draining fluid, swelling, and tenderness to the touch. The only lab tests that might be ordered would be a CBC (white blood cell count, which is usually high), there might be a differential (which is a count of the different types of white blood cells), which shows a "shift to the left", and lastly a culture.
Many possible tests. Blood tests that are commonly used may include a CBC or esr, a wound culture and sensitivity will be the best test in determining the appropriate antibiotic selection in case of an infection. Physicians usually use clinical judgment in determining whether or not the wound is infected or not.