Download Guitar/Bass Radius Gauges
Or, you can download the documents below for free.
Print them out. Double check the sizing guides along the side to make sure they're accurate and your printer hasn't skewed anything (there's a document for US-Letter and one for A4 to help prevent and resizing issues).
Paste them to some stiff card and cut them out.
Ta-da! Radius gauges.
A4 Radius Gauge Download
US-Letter Radius Gauge Download