Yes, coolant is mostly water anyway, but evaporation seems to be very small with it as it is relatively contained.
The main thing if you suspect a coolant leak is that it isn't a head gasket.
Simply checking the oil dip stick should be enough. If it really looks like normal clean engine oil and the level stays exactly the same when you check it in the same spot after the vehicle sat overnight, you should be ok. If it looks like a chocolate milkshake or oddly colored, dump the oil and give it a good look.
A blown head gasket should be fixed right away as you'll blow the engine if you are getting coolant in it.
As boss said, do a drain and fill every two years as a general rule. Makes servicing fast and easy, and you have much less scaling and corrosion as an added benefit which ends up being cheaper than doing it every 5-6 years and corroding your radiator and coolant passages, which can overheat your engine if the coolant can't flow properly in your engine, or gets so bad it causes a layer of insulation to form with so much scaling.
