Enter your coordinates on a Web browser (or a cell phone Web browser), and thebusschedule.com will tell you when intercity buses will depart in the main cities of Belize, Costa Rica, El Salvador, Guatemala, Panama, and Nicaragua. (It also covers Honduras, but you don't want to travel there presently due to the civil unrest.) While not a guarantee of reliable service, this website can help you plan your trip before arriving in country. It's worked for us on the ground in our testing.
(Thanks to reader Miguel Santana for the reminder.)