Baby boy or girl?

If you can see a small protruding white dot on the underside by the pelvic/ventral fins, it is a girl. But if your betta is young, then I'm unsure at what age the females begin showing their ovipositor, or often known as "egg spot". But if you see it the egg spot, it's definitely a girl .

Both young males and females will show what appears to be an egg spot. You can't sex juvies just by viewing one sign. You need to asses other as features.
1. Body shape - males are usually simmer than females. Males have smaller midsections compared to females (viewed from above - before feeding)
2. Males have bigger head and mouth. From above the head looks blunt compared to the more pointy look on female's head.
3. When they flare, males gills open +/- 90*. But then again young males often do not open that far if they seldom flare.
4. In general, male's fins are bigger than female's
5. Males tend to show brighter and more solid colors while females often are less vibrant and show breeding bars.