This is interesting. I was talking about this with my mum the other day - she asked me the same question.
There was a time where I couldn't even visualize it. It seemed so impossible, and I thought I'd probably be overweight forever and I, and other people, would just have to accept it.
But since I've been losing, I can visualize it most of the time, unless I'm having a real discouraged day/moment. But as I lose more I'm able to imagine it more. It's nice to imagine me more confident, more able, wearing nicer clothes, etc. etc. So I can imagine what I could look like.
But mentally I think it may take some time. Like...I can totally imagine myself "forgetting" I'm slim! I think there will be left-over "baggage" if you will. Being overweight can be hard to deal with, may just leave some wounds. I think I'll still think like an overweight person for a while.
Even now after losing some I sometimes feel just as big as before, even bigger! I think its because I just really am fed up with it now, and want it so bad that I sometimes feel even bigger...doesn that make sense?
But other times I'll look in the mirror, or feel my tummy and see/feel the difference and am able to say "yes, I have lost some, I'm doing good" It's quite a weird process!
I guess I don't know for definite until I am slim though...