You know you're fit when everywhere you go people start commenting out loud about how they 'should' take the stairs instead of the elevator...when the people around you unconsciously straighten their shoulders...when you can fit (even barely) into the Cruel Girl jeans you bought three years ago hoping to someday be able to wear them.
In the exact same way you were were so certain that a mistake must have been made at the factory and the size 14 stretch jeans you're about to pop out of are really size 4s.
You know you're fit when you touch your thigh and it's solid muscle and it's time to get new shirts because your shoulders are so broad.
You know you're fit when the pain of working out sounds far more appetizing than a chocolate shake and fries. Ickkkkkk! :P