The answer should always be the honest one. If she looks fat to you, then tell her. Conversely, don't expect her to think you look good when you've gained 100 lbs since you got together. Works both ways.
My wife knows I look at other women. I don't try to hide it any more than I would expect her to hide it when she cops a look at another man.

 

everyone looks....it's human nature

 

Manny, you're starting to post S2KFan style... 5 words or less at a time!