In order to find bras that truly, truly fit you, the wires need to enclose your breast tissue and not lie on it. This hurts and feels very uncomfortable.
So how do you find out how wide or narrow your root is and where your wires need to be?
One easy method works as follows: While standing, grab your breast, press it lightly to your torso and move it toward your shoulders. You can also lie on the back and move your tissue outward or just let it slide to the side. Where your breast naturally folds over is the point where your breast tissue ends. You can mark these points or lines with some liner (kohl/eyeliner works well) and put on your bra to see if your wires lie in the correct place. You can also use this method immediately after taking a bra off. If you have wire marks on parts of your breast, your wires are too narrow!
The above method may not work for everyone. If you feel like it doesn't seem consistent with how you experience wires, try to work it out by touching (comparing different kinds of tissues and how they feel), thereby figuring out where there still is breast tissue and where not. A lot of women have more sensitive breast tissue directly before their menstruation, this may make this method easier.
Also see our article on right fit - it's not just the wires which are important.