If the fog is from your heater core than it will leave a greasy film on the windows, and about the time the engine warmes up and coolant starts to build pressure you will just barely smell coolant.

This isn't a fool proof way to identify the prob but its a pretty good way to know where to start.

<img src="/forums/images/graemlins/barf.gif" alt="" /> <img src="/forums/images/graemlins/drunk.gif" alt="" /> mmmm Coolant