You need to go to a shop or get your hands on a cooling system pressure tester. You connect the tester to where the radiator cap goes. Be carefull not to go higher than the kpa of your particular system or you run the risk of rupturing the heater core. This test must be performed with the engine COLD. You can not do this on a hot engine. What you are looking for is a weeping hose connection or possibly water pump. Here is the reasoning. When you bring the car to operating temp, you presureize the system to the value of the cap. All hose connections and pump sealls, etc. are swollen and won't leak. As the engine cools, the connections relax and allow a slight coolant leak at any connection that is not properly tight. Coolant is forced out by the system pressure until pressure goes to zero. Now the system tries to go to a vacuum, so the connection has lost maybe 100ml of coolant and now stops leaking. This whole process starts over again next time you use the car. In a weeks time you have lost 500mls. Soon the car runs hot because it is low on water. Check this first, and then look at a head gasket. You may also have a restricted radiator and you are boiling off water since you are running straight water.

Good luck,
Peter

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