A fairly easy species to sex visually provided birds are mature.

The first place to look is at the size and shape of the bib. Cocks are bigger and more "triangular" shaped while hens are more "pear" shaped. This is more obvious when perched than in the hand (cock left)

Cocks usually have a slightly lighter coloured head, and his beak is usually a deeper red than the hens. This is the case in pure sub-species as well. Hens with deep red beaks do exist but this points to selective breeding rather than purity (cock left)

Also evident when viewed from the top.

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But the easiest way to sex them for me is by comparing the length of the two central tail feathers, you will find that the cock's are much longer (cock left)

There are two sub-species of the Long-tailed Grassfinch : The P. a. acuticauda from the eastern part of its range with the yellow beak (right) and the P. a. heck from the western part of its range with the red beak (left) Integrates occur in area of overlap.

There are also a few mutations of the Long-tailed Grassfinch around the world. Overseas there are Grey, Pearl and Pieds. Locally we only have the following 3:

Fawn which is naturally quite varient, thus the darker birds are referred to as Chocolate and the lighter ones as Fawns. This is the same mutation.

Some fawns are even lighter than normal. This is where the bird is both a White/In as well as a Fawn. Then it's somewhere in between. Given the sex-linked inheritance of both mutations this colour variety can only occur on cocks.

Normal fawn left and White/Ino Fawn right.

Then there is the Ino mutation wrongfully called white. The birds are off-White with a cream bib.

And then there are pure whites, unfortunately crossed with Ino's by those who don't know better resulting in very weak birds. True black-eyed birds are very rare. This one is the pure white lacking any colour to the bib. This one has Ino blood in it resulting in an albino.

Ino top, white bottom.

Sexing whites can be challenging, this is why I go by the tails (cock left)