As the chicken cooks you may notice that it tends to plump up. As the chicken gets close to being done this plumping goes away. Also the skin of the chicken will be a dark, golden brown. These are clues that it is nearly done and time to start checking the temperature.
To test the internal temperature, use a meat thermometer and take a reading in at least two different places. Test the center of the chicken breast and in the thigh right above the drumstick. These are the densest parts of the chicken and the slowest to cook. When both of these places read above 175 degrees F, it is time to take the chicken off the grill.