I know most of us don't like to do it, but... sometimes you just have to bite the bullet and swatch!! I rarely bothered, but my recent sweater knitting binge really drove home how important this little task is. But measuring with a ruler or tape wasn't easy, and the gauge I did have only measured over a 2" area. SO, necessity being the mother of invention, I went down to the shop came up with this gauge, which will measure a 4" square swatch, and the T shape makes it easier to count both rows and stitches!! Also on the gauge are metric and imperial handy 5" and 12.5cm rulers, as well as a needle gauge.
Laser cut from 1/4" birch ply, the gauge measures 5" square, and is sanded smooth and finished.