First clue, if it's a fairly bad leak, will be a lot of whitish-grey smoke from the exhaust, especially on start up. If the leak is such that the coolant can also get into the oil, check the underside of your oil dipstick or oil filler cap. If the coolant is leaking it mixes with the oil slightly and leaves a tell-tale creamy beige gunk. This can also be seen at the opening for the crankcase breather... it may be a few drops of the mixture on your garage or parking space floor. If it's more of a slight leak you may need a compression check, which usually can only be done at a garage... Good luck!