Head Gasket failure check?
 

I'm new to diesels and test drove 3 Cummins Dodges and bought the one that looked, drove, and sounded the best. But now I'm thinking I've got a head gasket leak. Here's my story:

Drained the oil in one of my well used oil pans and found a little bit of swirly chocolate looking fluid in there. Was thinking it was antifreeze.

Did a cooland system pressure check and it looses pressure pretty rapidly. Coolant is full however but I will watch for this because the truck is new and I put 200 miles on it so far.

While it sounds nice I noticed there is a bit of a shake at idle. I can tell you the antenna moves. I'm not sure if it's shaking more than it should or if this is common.... it shakes more than my old 460 Ford F250.

I'm suspecting a head gasket is starting to fail but not serious yet. What's the quickest, easiest way to tell if the head gasket is failing?