I checked for bubles in the coolant, the coolant in the filler neck by the thermostat was low, i ran it for about five minutes and got a lot of little bubbles. Are bubbles in the coolant a sure sign of a bad head gasket?

 

I was getting bubbling in my coolant reservoir when my 00 was hot. In my case the radiator cap was not sealing right. So I replaced it and the thermostat at the same time. No more bubbles. I believe one of the signs of a bad head gasket is unburned gas was getting into the coolant and collecting in the reservoir.