Well your cooling system for your car is one of the most important things on your vehicle, without it the motor will not work for very long at all. The cooling system allows the heat from the combustion chamber of the engine to disperse away from the engine allowing the engine to remain at a nominal operational temperature. The system is pressurized to allow the boiling point of the fluid to be elevated usually by about14PSI to gain about 25deg C before boiling will happen. The working temperature varies a little from vehicle to vehicle but in general the idea is to keep the cooling system temp well below the boiling point as this is when water changes form and simply will not work. Most cooling systems are not just water as they have another purpose which is to stop corrosion between dissimilar metals in the engine, so you will notice that most coolants will either be red or green in colour it is important not to mix the colours as they are not compatible. It is how ever OK to top up your radiator with quality clean drinking water.
To check the cooling system there is an over flow bottle under the bonnet, so the simplest test is if your fluid is disappearing from the over flow bottle then you have a problem. Sometimes you can see where its leaking or you can see fluid on the ground. If you cant then you need to pressurize the system so that the leak can be easily found. Remembering the fluid can leak out of the system or into the engine. A compression test will usually tell if the fluid is leaking into the combustion chamber.
They are many cheap testing kits on the market and its just a matter of attaching it to the over flow tank and then pumping up the pressure in the test unit and then seeing if the gauge holds pressure. Obviously if the gauge drops you have a leek. At Viva Auto Repairs we often leave them connected over night and if you have concerns to weather you have a leak or not bring it in and we will check it for free.
If you do have a leak its important to get it fixed the leaks never get better by them self and in most cases its a warning of something bad to come, so take the hint if the over flow bottle is losing fluid its time to act as a 50cent hose clamp will always fail to protect a $10,000 engine and they never fail on a cold day only the 38deg C days.