How a bra should fit
The band is parallel to the floor.
Look at your profile in the mirror. The band at your back should not be pulled up but sit parallel to the floor.
The straps are comfortable.
The straps do not slide off, nor do they dig into your shoulders.
The cups fit well
The cups should hold your breasts without any gaping, spilling, or discomfort.
The center gore lies flat
The center gore (the piece that connects the two cups in the front) should lie flat against your sternum without leaving a gap between your bra and your skin.
The wire does not pinch or poke
The wire follows the natural curve of your breasts without poking into it.
You feel good
This is the most important thing. If you don't feel good, it's simply not the right bra for you.