well, if you have an (infinite) plane, then you just have to test if the center of the sphere and the plane are less distant then the radius.
The plane will be perpendicular to an axe, so it will be in the form x = 3 or y = 5 etc.
If it's x=3 for example, the distance will be abs(Cx - 3) where Cx is the x coordinate of the center.

If you have lines or rectangles instead of planes, then it's far more difficoult, becouse you must discriminate the cases in which the sphere collides on an edge and the cases in whitch the sphere collides on an angle.
Possibly an easy solution is to consider each edge a plane and test all of them, but obviously this is not precise in the angles it will not raise a collision in some cases (anyway you can tell it's a feature: rounded rect collision algorithm :mrgreen

IHTH