Then give her a hug.
Don't do this if you are bad at reading people.
The alternative is to just be brutally honest and that way filter out friends who aren't ok with it. It might hurt a bit, but if you value this kind of communication over having many friends it could be worth it. Just don't do this at your job since you will likely get fired or managed out.
I found the inverse to be even more valuable. Finding friends who are not afraid to be brutally honest with you when the situation calls for it is difficult, but worth it so much. Just make sure you actually can receive this kind of criticism first without taking it as a personal attack.
in the stereotypical "does this dress make me look fat?" conversation, the person is (probably) not asking whether her partner think she looks fat. the actual content here is usually a request for reassurance. even if the partner does think she looks fat, "no, of course not" isn't really a lie; it's just satisfying the subtextual request.
if on the other hand, she says "I have a meeting with some executives today, does this outfit work?", she is looking for a literal answer and responding with reassurance would be unhelpful.