The best method to check for this is a non-contact thermometer, which will enable you to inspect if coolant is streaming through the heater core or not.
Ụzọ kachasị mma iji chọpụta nke a bụ thermometer na-adịghị akpọtụrụ, nke ga-enye gị ohere ịlele ma ọ bụrụ na mmiri dị na-agafe ma ọ bụghi.