You'd have to diff the package(sources) -- generally backports will try to be "conservative" in it's dependencies (not pull in a new libc or kernel unless actually needed). I'm not sure (never checked) how far backports go in actual backporting -- but in general they'll be less invasive. I expect there's probably a couple of small patches when needed (say to patch around a newer version of a library not being available if new functionality isn't strictly needed).
