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.