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By testing a person's DNA. It can be done at any age as well as during pregnancy. The medical profession involved depends on when testing is done. Most common would be Endocrinology followed by an OB/GYN but in these current times one can send a sample to a place on-line and get the answer. But it would be best to do so at a doctor's office so correct answers can be given. I have had Doctors wrongly guess my diagnosis by just viewing the signs and symptoms, so in my view only DNA testing is best. Ignorance of Klinefelter syndrome and its outcomes is very high in the Medical field, at least in the US of A. So I feel the best information comes from those diagnosed with the condition and Mother's who have raised children with this Syndrome.