Another way to verify, (though a little gross), is antifreeze is very sweet tasting. Just get a little oil on your finger and touch it to the tip of your tongue.....not a lot, just a little. If it tastes sweet it's antifreeze. As mentioned above, there are many ways for coolant to get into the oil, headgasket and cylinder head cracks are the top two.

You'd be surprised what you can tell about a car by taste......