Following on from this question Asking "what's the pinyin/character for X?".
I've had difficulty previously asking native speakers which tone I should use for a particular character if I can't pick it by just listening.
The problem is they don't understand what I'm asking in most instances.
How do I say "Which tone should I use for this character?"
In the past I've used "第几声" for which tone, but this doesn't seem to be commonly understood.
这个字/词怎么念? = How do you pronounce this character/word?
这个字的发音是第几声? = Which tone should I use for this character?
I have also used 第几声 and have had better success with it than anything else. The real problem that you're up against is that most Chinese people know this intuitively rather than intellectually. Many if not most native English speakers might be very hard pressed to tell you what a preposition was, even though they use them every single day. I think it's like that for most native Chinese speakers who aren't language instructors.
Have you tried using 發音 or 念 as in 這個字你怎麼發音 or 這個字怎麼念？
Sometimes, I think it's better to ask if it sounds like another character (by association).
E.g., 這個字像「心臟」的「心」嗎？
