First, I can't take any credit for this idea. It is the very first thing you learn in The Knitting Workshop by Elizabeth Zimmermann. It's her way of getting rid of the attitude that knitting a gauge swatch is a waste of time. This way, you end up with a usable item.
You start with a 16" circular needle in the size you think you will be using for your sweater. Cast on enough stitches to fit around it quite comfortably. If you are planning on a ribbing at the bottom of your sweater, be sure the number of stitches are divisable into the ribbing pattern - 4 stitches, for example, in a K2P2 ribbing.
Knit about two inches of ribbing. Work one round of stockinette incorporating increases making an increase every four stitches. Count the stitches to see if you have a number divisable by 7. If not, use the second round of stockinette to add the number you need. Continue on knitting stockinette rounds. Here is where I often mess-up my hat and the one for my Hurry Up Sweater is no exception. You would be surprised just how tall your head is from the bottom of your ear to the crown of your head before you begin decreasing. See how my hat is a bit short? I should have continued on for at least two more inches before I began the decreasing.
After I knit about 4 inches of stockinette, I transfer the stitches onto a scrap piece of yarn and "try it on". Yes, you would think that with this step my hat wouldn't end up short but I think I tend to not pull the "hat" far enough down over my ears.
Once you are content with the length, you can begin decreasing. I work a round of K5, K2tog. That is basically working a repeat of seven because you are knitting stitches 6 and 7 together. Next knit a plain round. Then K4, K2tog (knitting stitches 5 and 6 together). And a plain round.
Keep repeating the decrease and plain rounds until about half of the original stitches have been eaten up with decreases. By now you will have to be using DPNs or two circular needles or one Magic Loop (long circular needle). You will need to work decreases every round now. If you were to continue alternating the decrease rounds, the top of your hat would be pointy. By working decreases in every round for the second half of decreases, the top will lay flat on your head. When you do the last decrease, you will have only a few stitches left. Break the yarn and use a blunt needle to run it through those stitches and pull them up together tightly.
Wash your swatch hat and let it dry. Now you can use it to measure your gauge for your sweater.