Sorry, lost notifications to this!
When they're older I find it very accurate, but by then I can tell by their size. As younglings, it's a good indicator if paired with other signs.
I'm still having trouble with my 2+ month olds...
The blue looks male, and the black pied may be female?
Are you able to take pictures of them, it's a bit difficult to see. Preferably with them together.
@Miss Lydia