The rapper got to answer that very important question after his concert in Tulsa ... while greetin' some fans and signing autographs Friday night, an envelope was stuck to one poster -- which is where Post got the sweet opportunity.

You can hear the excitement in the guy's voice ... and after some well-deserved anticipation, Post's eyes light up, and he reveals ... the fans are having a girl!!

The future dad seemed to be still reeling after the good news ... because he was absolutely jumping with joy in an elevator afterward.

Post and his fan have something in common, now -- both are girl dads!

TMZ broke the story, PM is expecting his first kid with his GF  ... and he subtly announced her birth in June while on Howard Stern's show.

BTW, Post seems to be in better spirits and health at his shows -- as you know, he fell into a hole during a September gig and injured his rib, and then messed up his ankle at an ATL show this month.