Just knit a St st gauge for your hat. I like to use a needle one size smaller (or sometimes even 2 for the ribbing), but be sure to cast on very loosely. To insure a loose cast on I like to cast on on a bigger needle, or over 2 needles. I like ribbing to be nice and snug, but want the edge to stretch plenty.
As far as knitting your swatch in the round, that is often recommended and not a bad idea, but gauge can play funny tricks anyway. One way to do an "in the round" gauge swatch is to knit across and then, leaving a big loop and keeping the right side facing you all the time, go back and knit the row again. Keep doing that. Kind of like a giant i-cord. Add a few extra stitches because the edge stitches get loose doing this. I do this sometimes to determine "in the round" gauge, but then when I actually knit in the round my gauge may not match that anyway. So I have taken to not bothering (most of the time) to make an "in the round" gauge, I just work it back and forth. True it is sometimes a tad but I find it works for me.
I just read a thread on Ravelry about doing "in the round" gauge and some folks like to make a hat, or sleeve in the round to determine the gauge when doing an in the round sweater. It's more accurate than a fake "in the round" gauge swatch. But what about when you want to make a hat to begin with? Ah, there's the rub.