Yes, I want you to tell me how, without a doubt, you know someone's gender? Should be an easy question, right?
Since appropriate pictures are verboten here, you' ll have to try to stretch your reading comprehension and imagination. If he has a dick (penis, johnson, weewee, whatever term you prefer, he is a male, I.e. boy, man, bloke, guy, whatever. If she has a ***** (vagina, camel toe, cunny...agai, your choice), she is a female, I.e. girl, woman, gal, chic...
Hope that clears things up for you.