I use a set of sticky labels used by electricians. They come in little vinyl squares on spiral-bound plastic sheets with "2", "4", "6", "8" and so on, pre-printed. I can take an x-acto knife, lift one of these little labels off (1/8" x 1/4") and stick them to the side of the spool or on the underside of the reel foot.
Since I only use one brand of line, all I need to know is what test line is on the reel.
If anyone wants me to find out the name/manufacturer of these labels, let me know with a post.