One potential sign that a policy is a failure is when the government has to keep insisting that it's not a failure. Janet Napolitano is in Mexico and Central America:
Ms Napolitano denied the drug war of the US and Mexico was a failure but rather "a continuing effort to keep our peoples from becoming addicted to dangerous drugs".
This isn't really an answer. Any policy by definition is an "effort." The question is whether that effort is achieving its stated goals, and in the "drug war" it is really hard to find evidence to that effect. It is rather sad that Napolitano, whose job centers on monitoring such things, can't come up with a better answer.
No one knows where the decriminalization debate in Latin America is going, but once again the United States is isolating itself by refusing even to engage it. At the very least, it is incumbent upon the U.S. to provide evidence about which components of its policy are clearly working and which are not.
Given the statements by Otto Pérez Molina, the Latin American drug war is gradually transcending ideology. As a result, in the future I can envision some type of regional initiative, based not on decriminalization per se but a broad rethinking of Latin America's priorities and policy options. Presidents in the region are very far from consensus on the issue, but I can still see them coming together with discussions that the United States government still considers taboo. Self imposed isolation would be the likely result.