Do you like to swatch to socks? Uh huh. That's what I thought. "Why swatch when you can just start a sock then rip back if it doesn't work out." I'm probably not the first person to figure this out, but it's a nifty idea, especially if you like to use Judy's Magic Cast On. You can use the Patch Toe method too if you want, but this recipe for a sock swatch/spinning oil bottle bag uses the JMCO.
Turn at the purl row and sew the live stitches to the interior of the bag. Take 24 to 30 inches of sock yarn and twist it until it doesn't twist anymore. Fold it in half, letting it twist upon itself, then knot off the loose ends. Thread into the holes in the sides and knot the ends.
You can now check your sock yarn gauge and you have something useful for you or your spinning buddies.