How to tell if a scab is infected?

Drainage: Yellow drainage or pus = infection.

Infected/inflammed: Red, hot, swollen and drainage (inflammation) are normal signs of healing and in fact, not bad but are integral to healing. If infected, the wound should not improve, the patient have fevers, chills, and other systemic signs of infection. Culture of the wound should never be with a swab on the surface but a simple tissue biopsy. Routine use of antibiotics is to be condemned due to the risks.