Hi Tom,

I my opinion, for this mutation it is still difficult to check the wings or the tail feather... it is because it will have some 'yellowish' colour which it a feature of a female.

The best observation now possibly at their behaviour, as per cockatielluver mention, normally the male is more vocal and more aggressive compared to female...

If the bird is older you will also able to observed the tune of their calling as for male, they will have some specific calling which is not in female.

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Best regards,
Tanin Eh Boon
Malaysia Bird Community