One thing you can do that sometimes gives a clue to the head gasket is to drain a little bit of oil into a glass jar. If there is any coolant in there it is usually in the bottom of the sump (cool engine) and you can see it in the bottom of the jar. You can also just lift out the oil filter and take a close look at the oil on the filter. If it looks milky that is a bad sign.
94 318I, 96 740IL, 97 528I, 07 335I