When someone in semi-formal conversation says to me "It is X" and I disagree, would it be too rude to reply "I don't see how that's X?"
Saying "I don't see" (when you do see) is humility for the sake of politeness, giving the other person a chance to help you to see. But it can often backfire, and end up sounding snarky.
If you are confident in your knowledge of the facts, and you do not wish to be deferential in that manner, you could say "I really must disagree" and then go on to say how it is not X.
Frankness between peers is not rudeness.
It might be. That depends on the situation and the person you are talking with.
If you want to avoid possible confusion and getting someone mad at you, then you should use the phrase "Help me understand".
- Improves clarity by seeking a clarification
- Conveys the other person that you care about him and his opinion
You could say
Please help me understand why you think that "It is X"?
Could you help me understand the thinking behind your opinion?