Assuming you know how to make a measurement of distance ...

Put one end of your ruler at the center of the circle, and measure the distance to the circle itself.

Alternatively, measure the distance from one side of the circle to the other through the center of the circle, and divide that distance by 2.

Put one end of your ruler at the center of the circle, and measure the distance to the circle itself.

Alternatively, measure the distance from one side of the circle to the other through the center of the circle, and divide that distance by 2.