[header] How to gauge wire [title] Purchase a wire-stripping tool. [step] As it's impossible to gauge a wire with the insulation on, you'll need to remove this thin plastic protective coating before measuring the wire gauge. The wire-stripping tool is specifically designed for this task: it looks a little like a pair of pliers. 
The tool has a small hole near its tip, which is lined with a sharp blade to cut through and remove plastic insulation. [substeps] You should be able to find a wire-stripping tool at any local hardware store.