I think you can observe many different symptoms from a bad head gasket.
You should look for coolant in your oil, and oil in your coolant.
You should monitor the oil or coolant levels, looking for drops over short periods of time.
You should look for a cloudy deposit under your oil cap, radiator cap, radiator overflow bottle.
Whitish smoke from your exhaust could also mean you're burning coolant.
Take out your plugs and see if you notice anything unusual on the tips.
Before you screw them back in, shine a flashlight down in to each combustion chamber. Look for fluids.
You can also buy a kit from NAPA which will let you test your fluids for evidence of oil/coolant co-mingling.
If you have coolant or oil on your piston tops and you're trying to get accurate compression numbers, it won't happen. The fluids will help seal the rings against the cylinders.

Most importantly, you should try and diagnose why your head gasket blew in the first place. It can happen for many different reasons and not all of them are catastrophic. In fact, it's usually sort of a big sign that something is wrong. If your h/g is blown, and you continue to drive on it, you'll do more serious damage which will likely trash the motor entirely.

From first hand experience I can tell you - if you've lost power, you're losing coolant and/or oil and you have whitish smoke from your exhaust - your head gasket is toast.