95 Nissan Quest overheating

1995 Nissan Quest; 170,000 miles; v6 engine

My Quest began overheating and I noticed my gauge was pegged out on hot. My coolant looked rusty, so I flushed my radiator(after removing rusty sludge from my pitcock) and replaced with new coolant mix. It still registers really hot after warming up and coolant bubbles out the reservoir overflow for a while after I shut the engine off. I suspect the thermostat, but cannot tell where it is located. Is there a test I can perform first without actually removing the thermostat? Is this a job I can do myself, or do too many other parts need to be removed to replace the thermostat? Thanks for any help.