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 fans, so you can see if they move 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 or two of superglue. This print weighs in at only 0.77 g, probably placing it among the very lightest around. Admittedly it will flush more than that, but the total is still just 2.1 g.

See also https://makerworld.com/en/models/189554#profileId-209002