Ah, yes, if you get the queue popup that tells you what bosses are dead/alive before you accept or decline the invite, that you can use. But once inside the game won't tell you any longer.
That is a static popup I believe, using a instance hyperlink. One could parse the instance hyperlink data. Take a look in the StaticPopup related parts in the LFR, LFD and LFG lua files, also in the StaticPopup lua file itself.