To do a swatch in the round. cast on enough for more than 4 inches and knit across for the first row on a circ or dpn.
Then, rather than turning the work, slide the stitches to the other end and knit them again, loosely draping the yarn across the back. This way it measures your gauge for knitting every row rather than purling.
If you have enough yarn, you could also knit across, break the yarn, slide, and knit across again.