MathUtilsGetDistanceToPlane(Plane, Vector3) Method

Returns signed distance of the specified position from this Plane. If distance is positive, then position is in front of the plane (determined by the direction of the plane's normal). If distance is negative, then position is behind the plane. If distance is zero, then position lies on the plane.

public static float GetDistanceToPlane(
	 in Plane plane,
	 in Vector3 position
)