I think I have Marfan syndrome, but I'm not sure if I want to go get checked. Will it affect insurance?

Unknown. The new health care law says that insurers cannot deny insurance for "preexisting conditions". So if this health care law stays in place it will not be a problem. No matter what, if you may have marfan's you need to have it confirmed, and if positive see a cardiologist. With good care, pts with marfan's can have normal lifespan. Without care they can have cardiac problems.
Maybe. It won't affect health insurance under the new health care act in us, but it could impact the cost of life or disability insurance. It is still better to get it checked because of the treatable problems associated with marfan's. Check with your insurance broker about the latter two issues.