What are some ways to diagnosing a blown head gasket that you can do from home?

I don't have a leak down tester/air compressor. I was hoping to figure out some other ways to do it...

It isn't smoking, but it is also full of water and not antifreeze.

you pop the cap off of the radiator, and it bubbles, as if it is boiling, but it is overheating, so it could very well just be boiling...
(its on a 97 jetta, so it isn't really on the radiator, it is on the fill tank... VW's are weird)

no water in the oil
no oil in the water
no leakage down the sides of the block

any ideas?

thanks