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I was listening to a song and decided to check the lyrics to it.
http://www.kasi-time.com/item-74499.html
夜風に吹かれ 考える 好き 嫌い 嫌い 好き 繰り返す
Since there were no question marks, so it made me doubt if it was a question being asked in the lyrics. Also, when I did a dictionary search for でしょう
http://classic.jisho.org/sentences?jap=%E3%81%A7%E3%81%97%E3%82%87u&eng=
Some have question marks, while some don't.
The short answer is 'no' because all questions end in 「か」 in any "official" or "formal" writing. You just know when you see a question.
I never even learned how to use a question mark in elementary school. I was surprised to learn that it was regularly used in English when I started learning English in junior high school.
In non-official and non-formal writing these days, however, the question mark is used everywhere, especially when the questions are short as in:
「マジで？」= "For real?"
「[車]{くるま}で[行]{い}くの？」= "Are you going by car?" or "Are you driving (there)?"
Even today, we would not often use a question mark at the end of a longer question ending in 「か」 in an informal setting like a personal letter. It could look "funny" if you used one. Old habits die hard.
Nope, it's optional. The final particle か indicates that the sentence is a question, so that can be seen as the question mark of Japanese. In fact, adding a question mark when there's already a か can seem redundant.
That said, you'll find it used a lot anyway, just because sometimes people want to use it. But it is definitely a casual thing, so you'll only see it in casual writings, like manga. You won't find it in anything formal.
Edit: after reading the song text more closely, I'd also like to add that sometimes, as occurs a few times in the lyrics, it's necessary to add a question mark in writing to show that it's a question even if it doesn't end in か. In speaking it is clear when this happens, but not in writing. This still only applies to casual works, though, because formal sentences will be grammatically set up to indicate that it's a question without the need for a random question mark.
Spoken Japanese in particular tends to use intonation to indicate whether or not a question is being asked, regardless of the final particle. (If it's a question, the speaker's tone rises a bit at the end, just the way it does in English.) I'd assume that's what's being done in the lyric, or what the lyricist thought people would assume automatically. They could easily have used "?" (and should have, honestly ^_^b), but adding "か" would have thrown off the rhythm.
It also depends on context: In these lyrics, if the singer's been mulling over their feelings about someone else earlier in the song, it could very well be "I love them. I don't. I don't love them. I do." If they've been wondering how someone feels about them, it's definitely a question.
With regard to でしょう, since it's sort of a "guessing" word, it technically always leaves room for someone to deny the assumption of the sentence, but it isn't always used to ask questions, per se. "疲れてるでしょう" would be "You must be tired" or "I bet you're tired", but if you're using it to ask whether a house guest wants to go to bed soon, you'd probably raise the intonation at the end, and in writing, if you wanted to keep the syllables exactly as they were (and avoid adding か) you'd need to tack a question mark on the end to show that.
