Managing to detect a leakage of the cooling circuit is not easy, since the liquid evaporates quickly and leaves few traces.
First thing to do: try to detect the leaking area. You always need to work on a warm engine, i.e. when the cooling circuit is under pressure. This will let the liquid come out and help you detect the leakage.
The points that are mostly affected by leakages can be the junctions, the sleeves, the radiator and the HVAC system.
The sleeves are in rubber and tend to harden with time. This will generate cracks which will let the cooling liquid pass through. Also the fasteners used to tighten the sleeves can be one of the causes: if too tight, they may damage the sleeves.
Second thing to do: switch off the engine and check all the other components that involve the cooling circuit, the engine, the radiator, the sleeves, the water pump, the thermostat and the cast plugs, searching for other traces of leakages, rust or anti-freeze.
Should this operation not be successful and no leakage be found, you can try another method: put a clean piece of cardboard under the engine, then start it and let it run. Should there be leakages, the drops will fall on the cardboard, those allowing you to detect the area and narrowing your search. Once the area is identified, thoroughly check all the potential points of leakage. You may use a mirror to check also the least accessible points.