Check for any signs of water leaking to the ground and if you do locate a leak follow the path. There could be a leak from the drain plugs, water jacket freeze plugs, water pump seals, the coolent temp switch, damaged gaskets, or a small crack somewhere in the coolent system. Check all hoses to see if they leaking or loose at clamps and connections. And if you cannot find a leak anywhere. check the radiater core or header tank, if the leak is in the radiater you should remove and replace, or have professionally repaired. Still can't find any leak. Ckeck your oil for water or foam. And if you do see that it is here, it's most likely a head gasket leak.