Two good clues have already been posted.
If the fan clutch is the issue, it should only overheat in slow traffic or when idling. At highway speeds, you get enough cooling, usually, even if you had no fan at all.
Sometimes the exterior fins of the radiator will get blocked with grass clippings, leaves, paper, 'love bugs' or other detritus. This can occur between the air conditioner condensor and the radiator sometimes so you have to look carefully.
Some engines are really bad about holding an air bubble when you think you have them full of coolant. You have to be careful you get this bubble out.
An internally blocked radiator or a bad radiator cap can also be issues.
Last, if you have eliminated everything else, you have to consider an internal engine problem. If you have a cracked head or block, or blown headgasket, hot exhaust gases can enter the coolant and heat you up fast. You don't always see oil in the water, or coolant in the oil when this happens.
Some GM engines have some well documented 'weaknesses'. Which engine do you have?
I wish I knew how and where to do an FAQ on this subject, since there has been a lot of conversation this year about overheating.