How can check that a born baby is bisexual?

Answer 1

You can't. Although sexual orientation is clearly biological (as opposed to psychological), as studies of twins and birth orders has proven, it cannot be pinned down to a single gene or a single chemical pathway. The process of sexual maturation is complex. And given that this process does not begin until around age thirteen, an infant would not express behaviors that one could use to predict its sexual orientation in the future.