There is probably some hilarious answer to your question but, just like a human skeleton, you can tell by the ventral bones; they are an inch or so apart by hens and close together or touching by roosters.
The ventral bones are two long rib-like bones in the vent area. It is an adaptation of the human pelvic bone. The ventral bones in hens are further apart to enable eggs to pass through the pelvic area. Other obvious signs of gender such as legs, head, feathers and gonads would have been removed before the chicken landed on your plate. Can't agree with Einstein on this occasion. Female chickens are hens, males are roosters, cockerels, cocks or capons.
unless you bought it from a wierd organic outlet it is female. The reason being that capons (castrated cockerels) are now illegal in europe, and this was the only way to get good meat on a male bird. Chicks are sorted at the hatchery into females, which are sent off for fattening or laying duty, and males, which are literally thrown into a liquidiser and used for dog food.