With a ruler, or other suitable length measuring device.
Measure the diameter (the widest part of the circle). The radius is half the diameter.
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Well, if you know the diameter you divide that by two to get the radius.
Circumference=2*pi*radius
radius = circumference/2*pi
You can either measure it, or calculate it if you know the diameter (radius = diameter / 2), or the circumference (radius = circumference / (2pi)).
Divide the circumference by 2*pi will give the radius of the circle.