There's really no better way to see how well you'll get coverage with a phone until you get your hands on it. This usually requires a purchase, or a friend or neighbor who has got the phone and service you're interested in. The next best thing is checking your carrier's site to see if it has a coverage map (here are links to AT&T, Verizon, T-Mobile, and Sprint). A third option is SignalMap, a user-generated service that lets people search for and review cellular phone coverage by location.
It's about as simple as it gets: just search by ZIP code and take a look. If people have bothered to note where they get (or don't get) coverage, you can see it on a Google map. Each coverage report shows what service they're using, along with how good it is on a scale of 1-5 reception bars.
Frankly, I think you're better off checking the carrier's map since it shows you where towers are, along with where they overlap. That, and if you've ever owned two handsets in the same location, you're bound to notice that a lot depends on those little antennae.