How to check the frequency bands of 3G and LTE carriers to make sure they are supported by iPhone 4s, 5, 5s, 5c?
One place would be to look on Apple's own page here.

There is way too much detail on that page for me to summarise here, but it covers most (all?) countries and carriers where the iPhone is currently available, including those in the US.

So find any models that are supported by your current (non-US) carrier, and see whether those models work with any US carriers.