How many miles on the car?
Has it ever had the head gasket replaced?
Was it sudden?
I think a head gasket would tend to start slow and get worse, but I would'nt say that was a rule.
The first place I'd inspect if I thought I had a leak at the back of the head would be the heater return line. There's a heater hose that returns (I think) coolant from the heater core back to the rear drivers side of the head. It attaches to a steel nipple that bolts (2) to the head. The other end of the nipple pushes into the head about an inch or so and is sealed with an o-ring that hardens and shrinks with age. This could be the source, or it could be the hose itself.

Take off the air cleaner and you'll see it on the head back by the oil filter. It's a little tough to get to but use a flashlight and feel around it for coolant. Check the hose for cracks or hardness. If it seems to be the source, try tightening the hose clamp if you can as a temp fix. If the hose looks like it needs replacing plan on doing the o-ring as well and maybe change the oil at the same time since it helps to remove the oil filter for more room.

If this turns out to be the source I'll post some extra info/procedure.