A Needle Gauge (or gauge ruler) is a tool used for determining needle size. They come in various shapes and sizes, are often made of either metal or plastic, and some feature cut-outs for measuring gauge. The most common needle gauges available today have both US and metric sizing, allowing for easy size conversion.
To use a needle gauge, you simply stick your needle into each hole until you find the smallest hole through which the needle fits. The number underneath or above the smallest hole will indicate your needle's size.