Condensation visible under the cap is normal. If people ripped apart their engine every time you saw that stuff you'd be ripping apart brand new cars from factory. If you have a rainy day and/or live in a cooler climate, you'll notice it more often. Give the engine a good 45 minute drive with it nice and warm and if you STILL have that junk under the oil cap, it's a little safer to assume there's a problem. I would never use that as the sole sign that your head gasket is blown though, I'd check other signs such as actual measurable coolant loss, coolant in the oil pan, etc.