Here are a few tell tale signs that your headgasket is going/gone.

1: Overheating while driving
2: Coolant in OIL
3: Oil in COOLANT
4: Copious amounts of white smoke coming out of your tailpipe, think "Spy Hunter" smoke screen
5: Leaks at or around the head-block surface.
6: Extreme random consumption of either of the two fluids above.
7: "Chocolate Milkshake" appearance of oil cap and the oil in your head.

Any of those in your scenario?