How would I know know if I have marfan's 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.
Get Checked. There are many possible issues related to marfan and very specific criteria (ghent criteria) that need to be met. There are far too many potential issues to list here. Please discuss your specific issues with your physician(s).