How is impetigo diagnosed?

Appearance, culture. It often is obvious, but though many experts do not reccommend culture, often one is needed to see of the diagnosis is correct, and what bacteria, staph, strep or both is responsible, and what antibiotics will be effective.
Clinical, +/- test. Clinical: painful, red, itchy, poor healing lesion with honey-colored crusts test: swab for culture and sensitivity may be performed, biopsy rarely if other diagnoses are suspected.