Shooting in overcast and foggy conditions can be a huge relief at times but it also ends up being a bit trickier because it can be harder to figure out which light is good and which is bad until you bring the shoot in to edit. When I shoot in conditions like this I actually tend to shoot front lit a bit more than usual because that keeps the muddy shadows away from their skin and keeps the background a bit darker to show off the mood of the fog. Since the difference between front light, side light, and back light is so subtle in light like this, an easy trick is to hold your hand out in front of you and squint at it until it turns into a blurred blob. From there, just spin around and you'll be able to see how the light falls on it a bit easier. Also, you won't look like a fool at all when you do that (probably not true).