This custom-designed wooden needle and hook guide will tell you what size your needles are (just poke them through!), and helps you measure your stitch gauge in either inches or centimeters.
You can also use it to know what gauge a yarn is by wrapping the yarn between notches and measuring the "wraps per inch." Make sure the yarn not wrapped too tightly or loosely, and that all the strands lay right next to each other. Then use the table below: