There is no way to tell an adult bird's age just by looking at it. When they are still growing and developing you can more or less estimate a bird's age, but once they are matured there really isn't any way to tell. Leghorns are pretty good producers for the first few years. After that...
Hens get big combs when they are ready to lay eggs. It may take her a few weeks to feel comfortable enough to start laying. But when she does laying for you, she will be a consistent producer of medium to large sized white eggs.