I wish I remembered where I found this link. If you enter a United States zipcode, you get a list of radio station frequencies along with estimated signal strengths. Or you can enter a station's callsign to see its coverage in its principal city. The output includes the station's callsign, frequency, signal in DBu, and a link to the FCC website with station details.
Use the links at the bottom of the page to look up AM/FM signal strength by zip code, or by callsign.
Zip Code Signal at www.v-soft.com