Drain the oil, if it looks milky then its too late and you need to replace the engine as coolant has mixed in witht he oil probably from a blown head gasket.
If the oil looks fine then you may just need a new thermostat or water pump.
You can test by taking out or just replacing the thermostat ($20?) and seeing if the problem is gone.
Waterpump is more difficult but the easiest way for me is to run the engine with the thermostat out and see if the coolant flows very well. Since your car isn't running this isn't an easy option for you. I would hook a drill to the WP belt and spin the WP that way, be sure to spin it the right direction.