Manhood is an accomplishment, an internal need and quest, for both cis and trans men. When we acknowledge this, the necessity and intelligibilty of questions like "But how do you know?" fades away. Seeing this will help dismantle one of the structuring cultural approaches to transgender people: mistrust.
Eight months ago I pulled a bra out of the washing machine for the first time since breaking up with my college girlfriend and loudly coming out of the closet as a gay man a decade earlier. This bra belonged to someone I would have called my boyfriend a week earlier, before she came out to me as a transgender woman.