1.) Wifinder is a good site to find public wi-fi hotspots, and people can submit them as they find them during their road trips.
2.) Getting a wi-fi detector is an easy way to find a hotspot, because not everyone posts a sign that says that free wi-fi is available. There are a lot of companies that offer signal detectors. For example, the Mobile Edge MEASL1 is on a keychain, which is a good way to find a hotspot and measure signal strength when stopping in new towns along the way.
[Photo: Flickr | Florian]