This is a good question. Usually patients are given a sheet of instructions following any surgery with information on what to watch out for and when to call your surgeon. Infection is certainly one of the things patients must self-monitor.
The earliest sign of infection is redness around the wound. Any yellow or green discharge from the area is a red flag and should be examined by a health care professional. Fever, swelling, and hot, hot skin are additional signs and symptoms of local skin infection. The presence of any of these (and especially if you have two or more symptoms) warrants at least a phone call to your surgeon or a visit to a local walk-in clinic.
Even minor superficial (surface) skin infections can lead to a delay in healing, delay in rehab, and delay in return to normal daily activities and/or work. Deep or major infections can have even more serious side effects such as scarring, systemic (blood) sepsis (infection), and even death from toxic shock syndrome.
It is well known that people with diabetes, who smoke, and/or who have multiple comorbidities (other major health problems) are at increased risk of infection after surgery. If the description of infection here or the risk factors associated with infection describe you, seek medical attention immediately. Early discovery and treatment of a superficial skin infection can save you much grief later if left untended.