Randomly define a Cartesian coordinate system! Or remind yourself which way the axes go. Or just gratuitously reify a mathematical concept.

A 20 mm cube (or die) whose faces each indicate the axis of the coordinate system they are facing: -X, -Y, -Z, +X, +Y, and +Z. Additionally, each axis is marked as a stripe along one edge of the die, such that relative to the text, the stripe is a dot is in the bottom-left corner. Each axis has all markings in the same color.

The cube comes in “right-handed” and “left-handed” variants; right-handed is the same coordinate system used by OpenSCAD. The two versions are mirror images of each other except that the text is still in the normal reading orientation.

The Makefile (which can be accessed on GitHub or Thingiverse; PrusaPrinters doesn't allow additional files) will produce the four separate material STLs from the SCAD file; building the SCAD file directly will only make a color preview with (almost) no geometric features unless you modify the part variable.

Also on GitHub: https://github.com/kpreid/3dp-coordinate-system-die
Also on Thingiverse: https://www.thingiverse.com/thing:3862237