I have some unknowns in my stash, and as I am still a green horn to knitting, I am inexperienced with gauging what weight each yarn is without the product packaging telling me. Any suggestions?
I was Googling for information about determining yarn weight by wraps per inch and found this site. There is information about the WPI thing down at the bottom of the page but they suggest you also do a gauge swatch to help with confirmation on results. Swatches are probably the best answer, but I never make a swatch over 4" square (with a couple of stitches on each side that I don’t count in my measuring—edge stitches can be distorted), unless the yarn is bulky or bigger, then it might be a good idea because you have so few stitches you are working on that you would get a more accurate picture making the swatch bigger.
This site also has more about gauge and yarns. This may not be the last word on the question, but is a start in your pursuit of of knowledge.