Wraps Per Inch
This is a quick and dirty way of figuring out how heavy a yarn is when it doesn't have enough comprehensible information on the label. This can be especially handy when dealing with yarn labeled in a language you don't understand or with homespuns or put-ups with only yarn counts listed. Instead of remembering the approximate yardage of 100g of worsted weight wool as compared to a sport weight, or memorizing or doing the calculations to figure out what 2/20 means, you just bring along a ruler or a marked pencil and figure out wraps per inch.
Essentially, yarn weight has to do with how thick the yarn is; worsted weight, sport, DK, fingering, lace, cobweb, etc.. There's a lot of variation within the weights, but then there's a lot of variation in individual knitters. Range is what really matters. So, if you know how many wraps per inch is fairly typical for your favorite weight yarns, you can double check them quickly on the spot. The added benefit is that this works for any yarn regardless of its composition. Since cotton weighs much more than wool, going by yardage per X grams is not going to remain consistant, and this becomes even more complicated if you consider how many other fibers are involved in yarn production.
To measure wraps per inch is simple. Gently wrap the yarn around the ruler or something-or-other which has an inch marked off on it and line up the strands so they just touch. There should be a bit of tension on the yarn, enough to pull it smooth but not enough to stretch it taut. Wrap until it covers an entire inch and then count how many strands it takes to do so.
For example, recently the lovely Heisenberg sent me a cone of yarn for the latest secret santa. It was labeled 2/30 which refers to its yarn counts, a confuzzling matter of official standards and such. "2" refers to its plies which I already knew. The "30" however is fraught with mystery depending on what kind of fiber was spun. After some research 2/30 resolved into 3,840yds/lb. or 240yds/oz. Hm. Now, I've got a ball of fingering weight yarn sitting next to me which comes in at about 130yds/oz. Clearly this yarn is much thinner. This took me googling and puzzling for about twenty minutes through two articles on yarn counts, pondering whether the tag saying "worsted" meant what I thought it did (it did) and doing the math.
However, the first thing I did when I looked at the yarn (after admiring it under 3 kinds of light), was wraps per inch. In about 45 seconds I came up with 23wpi twice (which I take to be reliable double checking) and concluded "laceweight."
Of course, anything which is 23wpi looks laceweight upon first glance, but this way I could make sure my eye wasn't fooling me. I'd spent the week before working on worsted and bulky weight yarns so I couldn't rightly eye it. Now, yarn tends to bloom a bit off the cone, I expect it will be much closer to 20wpi if I skein it and let it relax but it will still be a laceweight.
WoolFestival.com - This article also has useful yardage estimates to help when trying to decide how much yarn to purchase.
To read more on yarn counts, see Yarn Forward's article on the matter.