The best way to tell if your chicken is done is to take its temperature using a digital meat thermometer. Digital thermometers take the temperature reading at the tip of the thermometer probe so they work well for both thick and thin foods.
A basic digital thermometer is designed to take a temperature reading in about 10 seconds and cannot stay in the food during cooking. Remove the chicken from the heat and slide the thermometer into the thickest part of chicken pieces or into the breast on whole birds. Check the temperature reading. If the temperature is too low, remove the thermometer and put the chicken back on the heat to finish cooking. If the temperature is right, the chicken is ready to eat.
For whole birds and other foods that take longer to cook, consider a heat proof digital thermometer. These have a base unit that sits on the counter and a heat proof cord that connects the temperature probe to the base unit. The probe stays in the food during cooking and the base unit displays the temperature throughout the cooking time. Slide the probe into the breast on a whole chicken. Connect the cord to the base unit. Turn the base unit on and follow the manufacturers instructions for setting the desired end temperature. Once the end temperature is reached, remove the chicken from the heat.
Digital thermometers are available in kitchen or restaurant supply stores. Expect to pay $30 or more for a good quality thermometer. A good thermometer is your investment in peace of mind.