You could always avoid thinking about it and just defer to the dictionary definition of gender, common fucking decency, and the visual and social cues presented to you rather than making assumptions about peoples genitals and genetics - or worse yet, demanding to inspect them for yourself.
Putting aside the fact that the consensus of the experts in the fields of lexicograpy, medicine, science, sociology, psychiatry all disagree with you, I need to ask why you’re motivated to work so hard to be such a cunt to people? What does it benefit you? When was the last time you interacted with a trans person… or have you been beaten into a hateful frenzy by the scaremongering of your reactionary pundits of choice over an issue that has no meaningful effect on your life?