Some kind of oil in coolant

I just bought a 94 Celica St w/ auto trans and 117k mi. Today my daughter went to check fluids... and the fluid in the resevoir was nasty, so I took off the radiator cap and stuck my finger in there and the fluid was dark and felt oily. It runs great, runs cool. No smoke no smell. How do you tell what kind of oil it is? Engine or tranny fluid? I found records on the car and found that in 07 at 90k the coil and head gasket were replaced. could it be that oil made its way into the coolant when the gasket was bad and it just wasn't flushed, or could it be a bigger problem? Where do I start?