Hey just joined this page I think my 2004 wrx needs a head gasket change. What the easiest way to tell and best place to get it done? 

Easiest way to tell is they normally pressurise the coolant system and force it out of the overflow bottle. That could be a air lock or faulty thermostat too ,so a sniff test for combustion gasses in the coolant system would be the best bet if you have those symptoms.

Otherwise a compression test would let you know if the gaskets blown between cylinders or oil ways .

As for the best place to get it fixed that depends on opinion and the area your in bud

I'd go with a compression test as well, what's made you think you got a head gadget gone