This is something I posted in another thread. I’m going to C&P the whole post to provide at least a little bit of context:
My question pertains to the second sentence. Was I correct in ending that sentence with a period? Or should I have ended it with a question mark?
I’m asking because at the time, I had a strong urge to end that sentence with a question mark. Despite the fact that I wasn’t actually asking a question.
Odd. I quoted that post and typed in a question mark in my similarly phrased response. Then thought if Grrr! thought no question mark, why not? (Or … why not.) My punctuation is truly bad so I went with the flow.
My answer is based on pure intuition, no academic sites that I know of. The grammatical structure of the sentence is declarative, but the intent and tone is very clearly interrogative. That is, you’re not telling someone to “guess” by completing a poll or filling out a contest form, you’re really asking the rhetorical question, “so what do you think the stupid cat did?”. And I say intent overrides structure, and the question – rhetorical or not – takes a question mark. 
“Guess” is an exhortation, not a question. No question mark.
Technically speaking, it is the imperative mood. So, yes, no period.
I agree, and I add that, if you ended it with a question mark, I would have heard it in my head as ending with an upward inflection, which would have sounded weird and wrong.
In a very narrow sense, sure. But as I said in #3, to any native English speaker, “guess” in this instance in colloquial English is semantically equivalent to “And what do you think [happened in the described circumstance]?” Question mark, absolutely.
Of course, some may prefer to stick to the same narrow interpretations that plague computer translations. Guess what I think about that? 
I agree with this. It’s a command. “Sit.” “Stand.” “Shoot.” “Guess.”
Now, when writing stream of consciousness, which is what a lot of posts essentially are-- and some of them I’d say even qualify as dialogue-- posters do get a lot of leeway in trying to produce a “voice.” If the poster wanted a question inflection, then a question mark would be permitted, I think, but to my mind, a dead-pan delivery fits the line better, and that calls for a period.
Try this out. For objectivity I’ve left out any ending punctuation:
Guess which one the cat puked in
Which one do you suppose the cat puked in
The first is the subject of contention. The second is explicitly interrogatory. They both mean exactly the same thing.
I would say them both exactly the same way, with an interrogatory inflection. I submit that if you say the first one with a declarative/imperative inflection, you would sound like a robot.*
)As mentioned above, questions marks have some flexibility, in that they are often used to indicate pronunciation (upward inflection) even though a clause might not grammatically be a question.
Also, sometimes the grammar calls for a question mark, when pragmatically an utterance is not really a “question”:***Would you close the door?***Depending on who says this–and to whom–it isn’t necessarily a question functionally, and it might not be said with upward inflection. Still, the convention is to keep the question mark, because grammatically it’s a question. A writer of fiction, however, might not use the question mark in such case, to emphasize that the speaker isn’t asking, but rather telling the other person what to do.
I did a little mini-corpus analysis by querying the incidents of the phrase “guess what” on this board. Of the first nine occurrences on the first two pages, two were terminated by exclamation marks, one with an ellipsis, and all the remaining six with question marks. None – zero – were terminated with a period, as would befit a declarative or imperative.
So it appears, at least from this tiny sample, that the way people actually think, speak, and write is not always the same as some of the micro-analysts here think they should. And, ironically, I’m the one who’s frequently accused on these boards of being a prescriptivist! 
Cite:
Also, keep in mind that there isn’t just one question intonation pattern. Yes-no questions, “information” questions, and rhetorical questions all take on different intonation patterns, and even then they can vary, depending on the what the speaker wishes to imply. Rhetorical questions can be pronounced with the same intonation pattern as indicative mood statements, and in this case, it seems that the sentence in the OP is like a rhetorical question, so it isn’t really necessary to use a question mark, in this particular case. Since grammatically the utterance isn’t a question, there isn’t much need to use one.
I just did a real corpus search (on COCA–the result can’t be linked, unfortunately), and it seems that it completely depends on the context of the phrase. Sometimes it is indeed meant to function in a questioning way, and a question mark is used, and other times it’s rhetorical, with no question mark. Problem is, guess what is used much more in speech, so there isn’t any punctuation at all. But here is an example from a journal:
So–rough guess–there are about half like this, and half with question marks.
Interestingly–which I didn’t consider–is that the phrase guess what is often used as a discourse marker in speech. (I didn’t count those, of course.) I don’t have the time to do a screen shot of the results–maybe someone else can, to get a more accurate count. (But if you do it, remember that spoken language doesn’t count, because the corpus itself is putting the question mark in.)
This is all getting pretty subjective, but I don’t think your example is very persuasive. It’s a narrative account that encompasses the quote, whose original statement type is embedded in the narrative context to the extent that it doesn’t even have quotes around it, let alone the obtrusiveness of a question mark.
I could write something like the following:
He rang the doorbell and she came to the door. As soon as she saw him she asked, did he have a present for her.
One would not intrude either quotation marks or a question mark on that narrative, IMO, despite the structurally interrogative nature of the concluding phrase. The OP example is the converse, interrogative in purpose but not in structure.
Of course one could claim that the proper grammatical form in my example would be “… she asked whether he had brought a present for her”, but such a person should not be writing fiction unless he was writing for the Japanese-translation market.
I don’t care what everybody else in the forum does. “Guess which…” is an imperative. It’s an order. The subject of the sentence is the implied ‘you’. It takes a period. Period. Unless you’re screaming loudly enough to require an exclamation point. But never a question mark. The sentence doesn’t actually ask a question. It compels one to answer a question.
I’m a native speaker. It’s not semantically equivalent at all. One’s a question, one’s a command. What you’ve provided are two completely different sentences, and they’re punctuated differently. Try this:
“How old are you?”
“Guess_”
“35?”
“Guess again_”
What say you, wolfpup?
I would have used an ellipsis …
Emphasis mine
… but that’s just me …
To me, they appear to have different meanings. With a period looks like “I want you to guess”, and with a question mark looks like “I want to know if you think guessing is a good idea”.
To me, it’s a clear-cut situation - period is right, question mark is wrong. Compare with “Get over here right now?”
I say that your example is contrived to use the same word (“guess”) in a completely different context. In your example, it really is an imperative and would definitely take a period (or perhaps an exclamation mark). It means “I’m not going to tell you; I want you to guess.”
In the OP example, “Guess which one the cat puked in” is functionally an interrogative (IMO) in the same way that all the actual usage examples I cited in #11 were interrogatives which the posters instinctively chose to punctuate with question marks, and in the same way that a spouse’s greeting “Guess what I did today, honey?” would be similarly punctuated as an interrogative in most writing, and not treated as a command to participate in a quiz show. To be sure, the query is rhetorical, and is just an artifice to introduce a topic of conversation, but its perception as a query seems to me to be an example of the linguistic concept of pragmatics, where statements are framed and prefaced in ways that introduce the least amount of dissonance.
Conversely, the second sentence in the OP’s example, “Go on, guess!” is clearly an exhortation. The implied logical flow is that the first statement, a query, remains unanswered, so an exhortation is made to answer it. Some other examples cited here, #13 (the journal quote), and #14 (my parallel example to #13), further the idea that it’s the context and intent that determines the sentence type and appropriate punctuation more than a strict interpretation of its grammatical structure, IMHO.