Jet engines normally have a spiral painted on the cone, so ground crew can see if they are spinning. That is exactly what I needed for my toolhead part fan, so you can see if it moves or not! This prints as a thin sticker (face down) with a tiny edge for helping to center it on the fan. Apply it with a drop of superglue. This print weighs in at only 0.47 grams, probably placing it among the very lightest around.

See also https://makerworld.com/en/models/190661#profileId-210448