You can go on Hofo and read threads with reports, you can go on the FCC Spectrum Dashboard and guess based on AT&T Mobility's spectrum holdings (although it's not always right, they have B4 in Northwest Lower that's not deployed, they are running B5-only), or you can use an app on your phone that shows you the band, but it only shows you what you're using, not what's available. The phones, in theory, will usually park on the higher frequency bands available, and drop to the lower frequency when the signal gets weaker, but that's theory, and practice doesn't always work out that way.
It depends on where you want to go. I also have a problem with them selling a phone that doesn't fully support AT&T's network when they obviously are able to make one that does.
Agreed. Something stinks here. I get that the Verizon one has to be different to support B13. However, it seems like the rest of them should be the same phone. And the pure edition most certainly should have the same band support as the AT&T version, since AT&T is actively using all of those bands (3 and 7 through global roaming packages, 2/4/5/17 domestically).
EDIT: If there's a Sprint version, it may have to be different than the global one, not sure how the 2600 bands line up. The third one on that list, the qa3, smells like a Sprint phone, as it (at a quick glance) appears to have the same additional bands as the Nexus 5 to line up with Sprint's PCS G block and 2600mhz spectrum.