My GE Fridge is icing

Question

My GE Fridge is icing up in the freezer. I had a GE tech come in and he glanced at it briefly and said it was the defrost heater. I looked online and learned that it could be either the defrost heater, the defrost timer, or the defrost thermostat. How do I know for certain it is the heater, and then how do I replace it if this is the case? I see no online directions on the defrost heater and looks like I could buy one for about $60.