You can take some of your coolant to a radiator place and have them test it. They will be able to tell you if there have been combustion gases mixed into the coolant, which is a sign of a failed head gasket, oil leaks etc.
Unlike "the good old days", it's not so easy to watch the fluid over the cores of the radiator, which would give you a good idea of what was wrong.
You could have an issue with the thermostat or water pump as well, so it might not be a complete disaster (head gasket would be a fair few $ to fix).