determine if new buffer ref is empty buffer ?

hi.
when switching between buffer refs using set method (user input), is it possible to somehow internally query in my external the newly referenced buffer to find out if it is 'empty' or not?
or at least determine if it has been referenced before in this session or not? or is that something i'd have to keep track of myself?
just wondering if the buf api has something cool i am missing.
thanks.