"Sex reveal party". No, that sounds bad, too. "Assigned sex at birth reveal party". You know what? Maybe we shouldn't make such a big deal about what sex someone is born as and let them tell us who they are as they figure it out.
in some cases the evidence of "gender" is inferred based on a blood test (the popular commercial tests I know of just looks for the presence of Y chromosome in the mother's blood and infers the baby is male if so, and if not that the baby is female), so people who throw a party based on a blood test would be throwing a "presence of Y chromosome" party ...