Of course, the best thing is to not ask the question at all; however, if you just have to ask a question...
The wrong question to ask:
"Do I look fat in this dress?"
The right question to ask:
"Does this dress make me look fat?"
~
Her:
"Does this dress make me look fat?"
Him:
"No, of course not."
Translation:
"No, of course not. None of your clothes make you look fat. You make you look fat."
~
If you ask the wrong question - then, the only honest answer that can be given is 'Yes' - because - well -
if you have to ask...