1997 Ford Taurus oil in the coolant

I have a 1997 Taurus 3.0 liter ohv that is getting oil in the coolant. I'm not sure if it is engine oil or transmission fluid, niether one looks contaminated (milky) on the dipstick. When the problem first started the car had over heated because a freeze plug had rotted through so I figured it was a blown head gasket but when I took the heads off the gaskets looked fine. I brought the heads in and had them re machined and put it back together myself. This seemed to fix the problem for a couple weeks but the transmission was slipping so I tryed some lucas transmission fix and that didn't help at all with the slipping but a week later the oil was back in the coolant so bad that it was seeping out from under the coolant resivoir cap. I took the car to a transmission shop and they say there was no fluid on the trans dip stick. This makes me lean twards thinking that the oil in the coolant may by trans fluid. As far as I know the oil way that could be is through the radiator, is that correct? How can I tell if it is trans or engine oil?