It usually says in each country on this page

http://travel.state.gov/travel/travel_1744.html

if there is a six month requirement. For the Phillipines, for example, it states:

U.S. citizens may enter the Philippines without a visa upon presentation of their U.S. passport, valid for at least six months after the date of entry into the Philippines, and a return ticket to the United States or an onward ticket to another country.

That page also lists travel warnings, worth reading no matter where you are going.