Use a meat thermometer to check the internal temperature. For a medium-rare burger, aim for 130-135°F (54-57°C), 140-145°F (60-63°C) for medium, and 160°F (71°C) for well-done. Alternatively, perform a visual check by cutting into the burger; it should have a slight pink hue for medium-rare, a warm pink centre for medium, and no pink for well-done.