overheating diagnosis

I have a 2004 Trailblazer whose water temp guage recently, (yesterday) went into the red zone There was no "check guages" light, I was close to home, so I continued to drive. When I arrived home I checked the coolant level, clutch fan, recovery tank and hoses. I ran the engine with the radiator cap removed to check the flow and it appeared to be OK. With the engine shut down all of the guages return to their default setting except the temp guage which returned to approximately 210 degrees. I checked the car 24 hours later and the guage indicator was between 100 and 210 degrees. How do I determine if it is a defective guage or sensor and how difficult is the repair? Thanks in advance.
:smokin:

First you need to check if the engine is actually getting hot.
Did it boil over or act hot or smell hot or low on coolant in rad?
A good scanner will show actual engine tempt on scanner readings?
Or one of these will help.
http://www.calright.com/_coreModules/common/categoryDetail.aspx?entityType=6&categoryID=66&gclid=CMrT9Lqk6ogCFRieWAodkzhWpA