Marfan's has lots of possible symptoms that make a person a little bit different. Some things are good, some are bad, and a few are really bad.

I am really tall and have the same arm length/height ratio as Phelps.
I went to to a Marfan's specialist and had my heart valve and Aorta checked with an ultrasound imager (same as pregnant moms). I'm fine. If your not so lucky -- they are testing a "cure" that is having fantastic results -- a particular blood pressure medication.

Why is Marfan's so complex ? The gene is known -- why isn't there a simple genetic test like for Cystic Fibrosis, Tay Sachs, etc ?
The reason is that the affected gene for Marfan's is ~1000 times bigger and can be affected in many different ways.
Not only that -- it is a dominant gene -- you get one from each parent, and in some parts of your body it is turned on, other parts it can be off -- this is called the "expression" of the gene and it seems to be random. So if you're lucky -- you can get the height and flexibility, but not heart problems.

The clinical diagnosis is very demanding -- if you don't have a family history or serious heart problems you will not be diagnosed -- probably a good thing is you ever want reasonably priced health insurance .

If you really want to know if your genes are affected by Marfan's, you can get an expensive genetic sequencing done. These tests don't really solve anything for you, but some severely affected Marf's are now testing their embryos to avoid passing it on to their children.

If you have any symptoms, get your heart checked -- it could add many years to your life.
Check out http://www.marfan.org/