The easiest method uses a Hindle sphere as described by Texereau. It is not necessary to have a sphere as large as or larger than the primary will be nor is it necessary to have a radius of curvature equal to the focal length of the primary. Almost any fast reasonably good sherical mirror can be used and it need not be perforated. The method requires that the sphere be offset and tilted so that the RC of the sphere coincides with focal point of the primary of the finished system. The cass secondary is then examined with the light passing at the edge of the sphere. You can examine only half of the secondary this way but assuming that the curve is regular and does not have any astigmatism this is adequate. This works for a conventional cass. The spacing needs to be modified though if a Ritchey secondary is being figured as the conjugates of the hyperbola need to be adjusted.If you need more info, feel free to contact me via e-mail.