High-order matrix shimming has proved useful for addressing B0 susceptibility issues but requires prior knowledge of the shim elements position. For rigid coil formers the shim loops are fixed in space and the field maps can be pre-measured in a phantom. Flexible or movable arrays used in abdominal imaging present a challenge since the element position is patient-specific. Here we introduce a marker system for rapidly detecting the element position prior to or during matrix shimming. Our tests show we can successful determine loop position to accurately generate B0 field maps in good agreement with experimentally measured maps.