190e coolant leak

I have an '89 190e, 2.6 with a coolant leak near the front left side of the engine. I pulled the engine cover and could see coolant running down the side of the oil pan. It was comming from somewhere on the front left side of the engine. I suspect the water pump, but could not see if that was the source. Other possibilities are a leaking water jacket plug, or a head gasket leak. I didn't see either.

Any suggestions on how to pinpoint the source?