I mean when I want to use it to make something, how will I know what it is for gauge purposes? Or do I just need to work it out as I go?
I use two meansurements – length and wpi. To measure the length, I count as I wind the yarn onto my niddy-noddy. I know that it’s 56" around, so I multiply the wraps by 56 and divide by 36 to get the yardage.
Here is a general Wraps-per-Inch chart (though they do vary, not set in stone by any means!):
18 or more wpi, approximately equivalent to lace weight, knits at 8 or more st/in, use needles size 00 to 2
16 wpi, fingering or baby weight, or maybe socks, knits at 6-8 sts/in, needles size 2 -4
14 wpi, sport weight, sport, knits at 5-6 sts/in, needle size 4-6
12 wpi, equivalent to worsted wt knitting yarn , 4 1/2 to 5 sts/in, needle size 7-9
10 wpi, equivalent to bulky or chunky yarn, 3 1/2 to 4 sts/in, needle size 10 - 11
8 or less wpi, very bulky, 2-3 sts/in, needles size 13 -16
And there’s a great chart here that lists the wpi of a bunch of commercial yarns, so you can substitute handspun in patterns that call for other yarns.
A good article about measuring wpi: Measuring WPI