It's normal to get that milky residue under the oil cap if you unless the car's been nice and hot for a good half hour, ESPECIALLY if it sits for a couple weeks between driving. Condensation will build up in the crankcase, it's normal, and it's not a bit problem as long as you burn it off completely when you run the engine. This is why lots of short trips are hard on an engine's life span. Condensation waters down the oil, your bearings start to disappear.
Do the compression test. It will tell you if a cylinder is leaking compression at all. This could be due to either the gasket or the rings, but if you're not noticing blue smoke out the back or your not blowing oil seals due to blow by, it's not likely a ring problem. To be sure if you find a low compression on a cylinder, try a wet test by adding a small amount (no more than a teaspoon is necessary IIRC) of oil to the cylinder, see if the compression improves.
Keep a close eye on your coolant. If there's no coolant loss, then the head gasket is fine. If the head gasket is leaking, coolant would be disappearing somewhere, whether it be out the tail pipe or in to the oil.
If you do notice coolant loss, check for a rough cold start and/or EXCESSIVE white smoke out the tail pipe. Some white vapour coming out the tail is normal until the system heats up because of condensation/moisture sitting in the exhaust. If you want to check what the vapour is, take a white ice cream bucket lid, hold it in such a way that you don't melt it, but catch the vapour on it from your exhaust. See if it's pink (or green depending on your coolant), if so, you've got coolant coming through. You have to collect a fair amount to get any sort of color out of the liquid though, so if it's a small leak, this won't likely work. Also, residual coolant sucked in to the cylinders and sitting there on a cold start will cause a pretty noticeable miss.
If it's leaking in to your oil, you can look for the classic milk shake dip stick. You can also crack your oil drain plug after the car has been sitting and not run for a few hours. See what comes out first. Coolant will settle to the bottom of the oil pan because it is mor dense than oil. If you get clear(ish) liquid out first rather than oil, you've got your answer.
Hope that helps.
