I'm not sure what you are asking.
Are you asking how to try it on to see if it fits? You'll have to slide the sts off the needles and onto scrap yarn. Try it on. If happy put the sts back on the needles and keep knitting.
Are you asking how to measure the width/circumference? If the sts are distributed evenly among 4 needles, lay two needles (sets of sts) flat and measusre the width. Double this number (because it is half of the hat) to get the total width/circumference.