In February 2014 Queerty, the online magazine and newspaper focusing on gay topics, ran a piece titled "Seriously Sexy Trans Men Make Us Say 'Mmmm!'" What's interesting and horrid about the article? The comment section, which displayed the pervasive lack of understanding of transgender identities within the gay community.
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.