You can just cast on with what you think will work and knit a few inches, let the sock be your swatch. It doesn't take much more time to knit that than a swatch. To mimic a swatch in the round, cast on about 8 more sts than the gauge/4" calls for, work the first row, slide it across your needle and drape the yarn loosely across the back. Then work the 2nd row and repeat sliding it back to the beginning of the needle. This way is like knitting in the round; the 2-3 edge sts will be very very loose, so don't count them in the measurement.