Well, I really can't guess what the next answer here is likely to say.
Like Ewood, I agree with TIE that the mere words "guess what," as a free-standing utterance, could just as well express a command (or simply a request) as a question.
It should be noted that the words "guess what" are
always elliptical, and therefore depend entirely on context for interpretation. Usually, when these are used as an apparently free-standing utterance, they are shorthand for something like "Guess what just happened to me?" or "Guess what I just heard about you?" Taken in this way, they of course pose a question, but contrary to what Ewood says above, the question is an information question, not a yes/no question.
In other circumstances, the bare words "guess what" might stand for anything from a polite request to a stern command. I have no idea what your majesty means to do with me, the courtier says, to which the king answers, "Guess what (and perhaps a Dukedom is in store for you)," or, "Guess what (or it's your head!)"
If the ellipsis is filled in syntactically as in my opening sentence above, there are still other interpretations. You can guess what some of these are. So guess what some of these are, already!
