what I think gives me away is the very very VERY concave area of my throat where my clavicle meets. I don't think being smaller or thinner is necessarily a dead giveaway - sure, if you're sitting in the CF clinic waiting room and you see a smaller, skinnier person walk in, you might think they have CF too - but just out and about on the street would be harder to make that assumption. As cystics ourselves, I think it's easier for us to spot one another. I don't think people as a whole would easily identify us as having cf unless they were really familiar with it. And as some others have said, not all cystics are skinny or short. So even though we all might share similar characteristics and can probably easily identify ourselves and other cystics as "looking like a cystic", the average layperson probably doesn't know the difference.
It's nice to hear so many of you talk about the CF voice - I have it too and that's on top of sounding like a 12 year old!! <img src="i/expressions/face-icon-small-happy.gif" border="0"> I'm a bit barrel-chested too (thanks for that image sevenstars).