Since knitting in the round most often involves Stockinette Stitch, which is made entirely with knit stitches in the round, that changes the way your yarn measures up.
Sure, you could just cast on a whole bunch of stitches and make a gauge swatch in the round that would be big enough to fold flat and measure, but that takes a lot of time and a lot of yarn.
Instead, you can fake a circular gauge swatch with two double-pointed needles (or a circular needle). Here's how it's done.
- Cast on enough stitches to give you a piece of knitting that is about five inches wide.
- Knit the first row.
- Slide the stitches to the opposite end of the needle, without turning the work.
- Knit the next row.
- Continue in this manner until the swatch is about five inches long.
- Bind off, or slide the stitches onto a stitch holder to make measuring easier. Then you can still rip out the swatch later if you need the yarn.
When working a swatch in this fashion, make sure you leave the strands very loose in the back so that the edges don't curl in too much and distort your measurement.
This is basically the procedure you would use to make an I-cord, except you don't pull the stitches tight when working this way.
This sort of circular swatch will give you a good idea how your yarn will behave in the round, while saving you a lot of time that would be better spent actually knitting your piece.