We purposely made a trip a few weeks before my daughter turned 3 so that she would be free. She is above average in height, but not huge, and she was very verbal and brave (rode everything she was tall enough for). I think if someone talked to her they would have assumed she was 3, but I don't remember being asked at the turnstiles (5 yrs ago, so maybe I forgot). She was not in a stroller at that point either, since we rented. At many of the character meals though, they would automatically charge us for her meal and I would have to get them to take it off. Even though she wasn't 3 yet, I felt embarassed at having to make them go back and take the charge off when she did eat, enjoy the characters etc... I'm so paranoid I figured they probably thought I was lying.
An aside. At Colonial Williamsburg, they did not ask for proof of age, but you had to get a free child pass for the little kids which involved making everyone who got tickets actually go up to a person and say that their child was under, I think 6 up there. Some people probably still lie, but it's much less innocuous than Disney's system. I'm surprised that Disney hasn't gone to that with the number of people who sneak their 3 and probably 4 year olds in. They have to know.