The female Common Kingfisher is almost identical in appearance to the male.
The difference between the two lies in the colour of the lower mandible. 
In the male, it is black, or in some cases, mostly black. 
In the female, it is orange-red, or in some cases, mostly orange-red.
Young Common Kingfishers have brown legs while adult Common Kingfishers have red-orange legs.

1st & 2nd photo: A male. Livorno, Tuscany, Italy, 23 September 2019
3rd photo: A male. Lago di Massaciuccoli, Torre del Lago Puccini, Tuscany, Italy, 21 September 2019