What is all wrong with my VW?

I have a 1994 VW Jetta with a 2.0L 4 cylinder... i know for a fact that the head gasket is bad, but i would like to know if the water pump could be bad and the thermostat as well. How could i tell if they are good or not??? I figure i might as well change all thats wrong instead of end up just fixing it all over again...