Loss of coolant because of a coolant leak is probably the most common cause of engine overheating. Possible leak points include hoses, the radiator, heater core, water pump, thermostat housing, head gasket, freeze plugs, automatic transmission oil cooler, cylinder head(s) and block.
Make a careful visual inspection of the entire cooling system, and then PRESSURE TEST the cooling system and radiator cap. A pressure test will reveal internal leaks such as seepage past the head gasket as well as cracks in the head or block. A good system should hold 12 to 15 psi for 15 minutes or more with no loss in pressure. If it leaks pressure, there is an internal coolant leak (most likely a bad head gasket but possibly also a cracked cylinder or engine block).
It is important to pressure test the radiator cap, too, because a weak cap (or one with too low a pressure rating.
Obtain a loan free radiator pressure tester from the local auto parts stores such as Autozone, O'Reilly or PepBoys. Follow their instructions and pump it an adequate air pressure 12 -15 psi or the allowances on the radiator cap. Observe the leaks and perform the repair.
Note: do not OVER-PRESSURE than the pressure allowed in the radiator cap.