How to check the frequency bands of 3G and LTE carriers to make sure they are supported by iPhone 4s, 5, 5s, 5c? Suppose you am living outside US &amp; you want to buy iphone like 4s,5, 5s,5c in US.

Then How can you check the frequency bands of 3G and 4G LTE carriers (in the place u r living) to make sure they are supported by iPhone 4s, 5, 5s, 5c?

So, Each carrier use just 1 band right? or 

is there any carrier that require your iPhone to suppose 2 bands?

also, are there 2 or more carriers that use the same band?

Quite confused.
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.