testing/ getting codes to determine whats wrong with the car on 2005 GMC Sierra 1500 HD

Do i need to have a device to get the codes from the compute to find out whats wrong? I heard theres a way to do it manually. Need to confirm that my timing belt is out. Is there any other way to know if the problem is the timing belt?what other problems have symtoms similar to the timing belt. thanks!!

Asked by for the 2005 GMC Sierra 1500 HD
buy a obd2 code reader.