I've had this issue with fly fishing leaders, and some fly lines too over the years. At least with fly leaders they make a gauge to where you can check thickness to see what it is. And newer fly lines have the info laser etched right into them. I still use a small piece of masking tape on the inside of the reel to write down what it is.
Now that I'm beginning to develop quite a collection of conventional fishing tackle, it's becoming a problem remembering what like I have on various reels. How do you all keep it straight? Just better memory than me (which is entirely possible)?