At what time do you normally discover that someone has kleinfelter's?

Varies. Klinefelter's syndrome can be detected during anywhere from pre-puberty to adult age. Sometimes the first manifestation is infertility work up in adults.
By pediatrician. Boys/adolescents with klinefelters have very very small testes. The diagnosis is not difficult to make. Once it is suspected a simple blood test with clinch the diagnosis. Unfortunately i make the diagnosis in adults presenting to me for infertility and zero sperm in their semen. As the condtion is associated with very low testosterone the diagnosis should be made sooner rather than later.