How do y’all predict genders?

So I’m not trying to be rude I’m just genuinely curious because I feel like y’all know something I don’t. People keep posting their ultrasound pictures with the side view of the whole baby asking what the gender is. Is there something other than a penis or vagina that helps y’all tell the gender?? I read a comment one time about the head shape? There’s some on here that’s actual gender reveal photos that people second guess and stuff and there’s also posts where they just want predictions/guess but I’m talking about the ones where you clearly can not see the genital area and people will ask what gender it is and say they’re so confused and need help. I’d like to know the skull thing if anyone knows or if there’s other ways y’all can tell?