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
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.