Question

How to find out which plane in a cubic structure has the highest density?

Answer 1

Close packed planes also typically have the highest d-spacing. So the first peak that one gets in an XRD pattern of a cubic crystal is from the plane with the highest d-spacing, which translates to close packing. This is true for pure metals in SC, FCC or BCC lattices. This may not hold true for compounds with those lattices.