Capillary optical fibers with hole diameters of several micrometers are important for novel plasmonic applications and medical diagnosis. In order to ensure the optical functionality of these fibers, the diameter of the capillary hole needs to be realized with high accuracy. Here, we introduce a novel and noninvasive methodology to characterize optical fibers and discuss it for the assessment of capillaries. To this end, the diffraction pattern produced by a coherent side-illumination technique of the fiber is analyzed. A numerical parameter retrieval allows us to characterize the capillary hole diameter with an accuracy of approximately 100 nm for radii in between several hundreds of nanometers and several tens of micrometers.