I tend to do a lot of holding up pieces and measuring them against myself (if I'm the recipient or anywhere close in size to the recipient).
You can use a large tapestry needle to thread the live stitches of your partially completed legwarmer onto a length of yarn. And then wrap it around your leg to gauge the fit. Generally, your seam will take up about two stitches, which you can easily pinch together at the edge.
If the fit is A-OK, you can move the stitches from the yarn back to the needle by slipping them without twisting them. Then carry on with your knitting.