I'd know from the symbol in your posts.
That's specifically why the writers of message board software offer users the option of indicating gender in posts.
That is no guarantee either that the person is male or female.
Case in point... A girl from my community was expecting and the doctor's did all the test... Did them 3 times to be absolutely certain what the sex of the baby was going to be. Well the test they did kept revealing boy. We gave her a baby shower for a boy. Well a few weeks ago she gave birth to a 8 1/2 pound bouncing baby girl.
I mean it wasn't like they did one test... They did several and still got it wrong.
And when it comes to filling out forms. It is always possible to check the wrong box.
FireRose