All they really need to do is flag their visas and not let them in.
“All they really need to do is flag their visas and not let them in.”
...but there are also Americans. As it is, when a person enters the US, legally, the guys at Passport Control already know every country that person traveled to (again, assuming legally...but slipping in and out of Liberia would not be easy), so identification of the people is VERY EASY.