Yes, if Michael Phelps believes that there is a legitimate reason for him to be worries about Marfan Syndrome then he is probably at higher risk. Physical characteristics are part of the symptoms for the disease. Thus, it may be smart for him to be tested for the disease as those who are at higher risk are most likely to come down with the illness.
He's an elite athlete, so it's probably safe to assume that Phelps has been given a barrage of physicals and seen a variety of doctors throughout his career. He's been through training and a lot of testing, and I'm guessing blood work as well. I'm not sure how Marfan's is diagnosed, but I'm guessing at least one of the many doctors he's had to have seen would have considered it if he/she thought it possible.
Michael Phelps has a body that seems genetically designed for efficient swimming. It also leads many to wonder if he has the incurable condition of Marfan Sydnrome. When the issue first arose more than eight years ago, Michael Phelps assured concerned fans that he was disease-free. Regardless, many people with only limited knowledge about the disease continue to raise the question.
He is tall and lanky but Micheal Phelps does not exhibit the classic signs of Marfan Syndrome, so it is not something he should be worried about. Addtionally he constantly is being checked by doctors, so it is likely that if he had it, he would have already been diagnosed.