Rear main seal or heads gasket? high temp!

[fongo]

Hi everyone.
Ok, so here's the problem.
I recently have been having surging high temp (all the way to the 270 reading!). It climbs shortly after 10 or 15 minutes running the car at idle. When I went for an oil change, the guys told me that I may have blown my heads, head gaskets or rear main seal, since the oil was coming out like chocolate milk! They said its most likely oil and coolant mixing as a result of the leak depressurizing the coolant system causing no circulation and hence no cooling. However after the oil change I looked under the car and didn't see any drippage.
How do I know which one is the problem? How do I effectively diagnose it? Can't make it to the shop because it starts to overheat 2 blocks away from my garage! I need to know because I'm planning to work on the car early spring.
Thanks!