what would cause coolant loss with no external leaks?

I think if I ran the engine the oil would be milky, when I checked the oil the level was very high on the dip stick but not milky. this was after the vehicle sat for a day or so. I also have a loss of coolant, but not external I don't want to run the engine if the oil is contaminated with coolant. Could the problem be intake manifold gasket failure ,or head gasket? What test would determine the problem? Thank you for your expertise Mert