can't fill gas tank

I have an '02 Blazer that has developed a gas tank problem.
Has anyone experienced this problem before where a vent is
blocked from overfilling the tank and now gas won't go in. Once
the neck is filled, the air has to escape back out the fill tube
causing the gas to go in very very slowly. It's like turning a gas
can upside down all at once without opening the vent. Air has
to go inside the can at the same place gas is coming out. How
can I tell which vent is blocked?
Thanks, Bruce