I think it’s at about the 20 week scan if bubs wants you to see.

Unless you need to know the sex because you have a family history of sex-linked genetic problems, it’s best to wait until your 20 week anomaly scan. The chance of getting the sex right then is a lot higher. Most pregnant women find out their baby’s sex (if they choose to know) during their midpregnancy ultrasound, which is usually done between 16 and 20 weeks.

Usually at three months, though it can depend on how the baby is positioned. And I believe blood tests can tell you earlier, though I’m not sure why you’d want a test just for this.