One design is a small frame with several evenly spaced fine wires attached.
The pattern is compared to a mathematically generated diagram (usually done on a computer today) of what it should look like for a given figure.
Inputs to the program are line frequency of the Ronchi grating, focal length and diameter of the mirror, and the figure required.
The figure quality of a convex lens may be visually tested using a similar principle.
The grating is moved around the focal point of the lens while viewing the virtual image through the opposite side.