I hear you...Jack's way is good but some times you want to know without stripping line...plus you might want to know the brand, etc.....If I have lost the one that came with the line I use duct tape and write on it used to put it on the spool inside the reel but because of a fairly recent tip here I now put it on the undersurface of the reelfoot...easier to access.
I use something similar to Jack's for the line because if the rig is strung you don't want to have to mess with the reel...You just don't know brand,etc.