"I have a very nice 96 outback

. except it is overheating

. Starts and runs great for 6 or 7 miles. Then the temp spikes. There is no knocking or pinging during acceleration. The coolant appears to be solid green (no brown from exaust like my last bad head) How can i tell if it is a water pump, clogged cooling system, radiator, or the expensive head gaskets. Any advice as to where to start? I can't afford a mechanic to do a series of tests. I can do the preliminary tests. Where to start?
Any advice would be welcome