Is it possible to detect which frequency band the iPhone is using I am currently travelling in a remote area. My iPhone 5S can see my carrier with good signal strength, and I can access 3G data. My wife's iPhone 4 can't even see the network, even for voice calls.

Both phones were purchased unlocked from the Apple store.

There is obviously some difference in the capabilities of the different generation phones. What are these? Is is possible to detect which frequency band the phone is currently operating on?
Not direct software solution but on the back of your iphone there are is a Model number, on my Iphone 5s it is A1530.  Different models are supplied in different countries according to the frequency bands licensed there. If you google that number with model of iphone and or frequency band you will find information as to what frequency bands your specific hardware supports.