Important note about "か" usage:
It is most commonly attributed to CASUAL exchanges.
You see...in any country, questions are confrontational in nature, because they require engaging another person.
This being said, if you have many or even a few questions, "か" is usually implied by increased intonation when
finishing a sentence.More often than not, this is the preferred method of questioning, because using "か" for every
question sounds accusatory...i.e. rude."か" is used repeatedly in things when you are generally questioning the listener's
intelligence, and it is usually quite offensive, not good.
The best way out when you have a question and you're in a FORMAL setting is to admit that you don't understand:
"すみません、わかりません。もういちど、おねがいします。"
(I'm sorry, I don't understand. Once again, please?")
or, if you do understand: (In this case, "か" is used to signal an additional thought and relative permission for it).
"すみません、わかります。でもね、あなたが訊くますおねがいしますか?"
(Excuse me, I understand. But may I ask you something?)
[I think the second part is right, but anyone can correct me if they think they've got a better way to phrase it.]