Several websites have TV guides for antenna channels, including TVGuide.com. Get there by clicking Set Your Provider. Start in the pop-up that comes up by choosing the ZIP code within the United States you live in, then choose Antenna for the TV service in the household.
Typically, there is only one OTA broadcast option, but depending on where you live, there may be two or more. Once you've picked one, it should bring up all the OTA channels in the area for a two-hour span. Zap2It.com operates in a similar manner but only offers the major channels of a metropolitan area.