I've used self-adhering file labels or mailing labels before for that. I write very small and very neatly

Grand
5 WF

or something along those lines, then trim it down to a very small size and stick it on the frame side of the spool somewhere. The stickiness lasts just fine in spite of an occasional dunking and the ink stays as long as it's ballpoint or sharpie or something else that doesn't run when wet.