Adam,
Even the forecasts which we as pilots receive, can't predict fog accurately. 99% of the time it will say "PROB30" or "PROB40", which just means there is a 30 or 40% chance of whatever weather phenonema follow's that (in this case, fog). Weather is hard to predict.
Last week, I had an early morning flight to a place in the Gulf of Carpentaria. Upon reaching it, I found unforecast fog (it's generally not good when you're down at the minima of 500ft and you're still not even in it). Despite me reporting to Brisbane Centre that there WAS fog there, and plenty of it, the ammended forecast still only said "PROB30",
.
So to answer your question, there is no sure fire way to know whether fog will happen or not.
Mick