Just a simple extruder indicator I thought of today. I just took an SVG, extruded it in Fusion360 to 3 different heights and colored the different layers.
If you print this on a printer where you can change the filament during print (gcode M600 for Marlin firmwares like my Prusa MK3) you can change the color after 0.4mm and 0.8mm from white to black to red. Otherwise use paint :-)
4 layers of each color when you print in 0.1mm layer heigth, 2 layer per color for 0.2mm layers.
There is a spot at the back to indicate the center. Glue a common 8x3mm magnet there and you're ready to go.