Check the core plugs in the side of the cylinder block. Check all hose connections for tightness, but ensure that the clips are not cutting into the rubber. Look along the hose for cracks, especially at points where the hose is flexed by engine movement. Inspect the radiator seams for splits, also the catch tank and its pipe.
Watch for leaks at the water-pump bearings and from the pump gasket. Check the thermostat housing for cracks, and the housing gasket for leaks. A rising temperature gauge, a pool of coolant beneath the car and a drop in the radiator level are signs of a leak. So are rust-coloured stains in the engine bay. But if you cannot actually see the leak, a systematic check is called for.