Adds a radius or diameter measurement to a curve.
Access this tool from the Locators palette:

Measure Radius tool workflows
Select the tool.
Click the curve you want to measure. You can move the new measurement while the mouse button is down.
The locator appears and shows the radius of the curve at that point. When you release the mouse button, the measurement is attached to the curve.
A point (cross) indicates the center of curvature (that is, the center of a circle having the same radius as the curve at the position of the locator).
Do one of the following:
and click off the measurement with the
to toggle the display of a leader between the center of curvature and the curve.To see a continuous comb plot of radius values along a curve, use Locators > Curve Curvature
with the Plot Value option set to Radius.
To see radius values at specific locations along the curve, Shift-click the curve. Shift-click a radius callout to delete it. Radius values can be displayed this way regardless of the Plot Value option you have selected.