Hi,
In my long-ago youth, I worked for my vet (dogs, cats, birds, not farm animals), and we did sexing of pet birds right in the office. The vet would trim a toenail just a tiny bit too short, catch the resulting drop of blood on a pipette, and could tell the gender from that. With a chicken, I'd think you'd have to be careful about infection with that method, since that toe would be walking around on the possibly dirty ground until it healed over (which doesn't take long, but neither does it take long to get an infection!).
The answer to your question, however, is yes, gender can be confirmed by a blood test. You should be able to take her to your local small animal vet to have this done if you want to.