Telling the sex of Goldfinches is a necessary skill if you are going to keep accurate records both as a licensed BTO ringer and also as a keen amateur ornithologist with a scientific approach to your hobby. The bird above, photographed from my garden hide earlier today, is a female. Generally, males have a more vibrant red mask on the head but this is not 100% accurate due to age condition and individual variation. However, one of the diagnostic features is the colour of the bristles protruding on to the beak, brown in a female and black in a male. At first glance I would have called this as a female but a close up of the head shows the colour clearly especially when compared to a certain male (see below).
In addition, this bird can be aged as a "first winter' individual hatched this spring/summer. This is based on the colour of the wing coverts which are brown as well as black. An adult would have a complete set of black coverts
Now if you look at this photograph of a certain male you can clearly see the difference.
The bristles protruding on to the beak a certainly black and it is a good example when you compare it to the female example above.
Interestingly, the white marks amongst the black mask are pin feathers and indicates that this bird is still undergoing a moult.