I usually think of them as easy to tell apart even when very young, but my little one, now 5 months old, had everyone stumped until she was at least 4 months old.
The breeder and I thought she was
probably a male, although I was never too certain. I think just about everyone here thought male, too. But she is now definitely female, going by behavior (no talking, loves to bite/chew), as well as an almost-white cere.
As a baby:
And at about 4 months (sorry it doesn't show here cere better but she is female for sure)