What is the test for a court to determine whether a child is in the custody of one parent and not the other parent?

the child, the court need not reexamine that question. Instead, it should preserve the established mode of custody unless some significant change in circumstances indicates that a different arrangement would be in the child's best interest." (Burchard v. Garay (1986) 42 Cal.3d 531, 535; accord Montenegro, supra, 26 Cal.4th at p. 256.) Put simply, "a child should not be removed from prior custody of one parent and given to the other '"unless the material facts and circumstances occurring subsequently are of a kind to render it essential or expedient for the welfare of the child that there be a change."'" (Burgess, supra, 13 Cal.4th at p. 38, quoting In re Marriage of Carney (1979) 24 Cal.3d 725, 730.)