How can you tell if you have Marfan syndrome?

Having the features. Features suggestive of marfan syndrome include: above-average height, long arms, long fingers, flat feet, high-arched palate, narrow face, crowded teeth, small lower jaw, pectus carinatum (pigeon chest), pectus excavatum (sunken chest), hunchback (hunchback), scoliosis (crooked back), hypermobile/hyperflexible joints, etc... A person with several of these features can see a doctor for evaluation.
Marfan. The syndrome consists of a collection of symptoms which include tall stature, long fingers, hyperflexible joints, and stretchy skin, along with other features. You would want a check up by your doctor or perhaps a geneticist for further evaluation.