You can use Plane Coordinates if you want to know the UVW coordinates of an XYZ point in a specific plane.
If you only want to measure whether a point is above or below a plane, then the Plane Closest Point component can help as well. It has a distance output which will be negative if the point is below the plane.
@DavidRutten approach is much easier, but just in general: Usually “left-right” checks are done by evaluating the dot between a plane normal and the vector from point minus plane origin. Might be a bit overcomplicated here against "plane closest poin"t, but f.e. in scripts this isn’t a big deal.