The best way to find out about carrier coverage is to get hands-on with a phone. We understand that not everyone has access to test devices, however there is always the option to check with friends and or family members and ask about their experience. Or possibly, grab their handset and do a quick speed test. Assuming none of those are an option, there is always the carrier coverage maps and third party reports.
We would suggest looking at the third party reports before the carrier maps and that being the case, it looks like Root Metrics has recently offered a fairly in-depth look at the state of LTE in the US. Their report covers the four big carriers -- AT&T, Verizon Wireless, T-Mobile and Sprint. The report took at look at a total of 77 markets and to give a spoiler -- it had Verizon Wireless with the most coverage and AT&T with the fastest coverage.
Needless to say, while fastest is generally nicer when it comes to the web, we cannot overlook Verizon as offering the most. This report notes AT&T as having the "fastest pure LTE speeds" showing LTE download speeds of 18.6 Mbps and upload speeds of 9.0 Mbps. AT&T had LTE coverage in 47 of the 77 tested markets. Looking outside LTE and AT&T was shown to have speeds of 4.3 Mbps down and 1.1 Mbps up.
Verizon on the other hand was showing average download speeds of 14.3 Mbps and average upload speeds of 8.5 Mbps. Stepping outside of the Verizon LTE areas and the average speeds were pretty far down as compared to AT&T. Verizon was shown to have averages of 0.9 Mbps down and 0.7 Mbps up. Just for comparison, this report notes that Verizon has LTE in 475 markets with AT&T having 135. Needless to say, we expect those covered market numbers to change. And in fact, they likely have already as both carriers seem to be adding LTE on a fairly regular basis.
Otherwise, T-Mobile and their HSPA+42 network was shown to produce average speeds of 7.3 Mbps down and 1.5 Mbps up which had them labeled as continuing to impress. While T-Mobile is certainly not comparing in terms of LTE speeds, they are doing rather nice when compared to the non-LTE networks of Verizon and/or AT&T. Bottom line here, we go back to the bit about actually seeing coverage in your area first hand, but this report does give a fairly nice look and should be helpful especially for those who regularly travel across the country.
[Update] It looks like some changes have indeed been made since this report was published. We have received word from AT&T letting us know they have since launched in 26 of the remaining markets. Basically, of those 77 tested, there is now only 4 where Verizon has LTE and AT&T does not.