What exactly is a "coordinated" turn?
It's important not to overthink it.  I'll keep it simple.  A coordinated turn means you are keeping the tail lined up with the nose in the airstream.  If you are uncoordinated, you are flying sideways in the airstream to some degree or another; the side of the fuselage is being presented to the airflow.

If you learn to fly in gliders, it's obvious because you have a yaw string on the canopy to tell you that the tail is lined up behind the nose in the airstream, since the string itself allows a direct visualization of the airstream.  In power planes with a propeller up front, you are forced to use an inclinometer to tell you the same thing, because it detects the lateral acceleration relative to the vertical axis that is present if the tail is not lined up behind the nose.

Practically speaking, whether you are banked in a turn or in level flight, it really doesn't matter.  The fuselage is lined up in the airstream or it's not.  The ball is centered, or it's not.  Just use your feet to keep it lined up as required using the skid ball.