There are several ways to test convex surfaces.

You can test through the glass. The problems here is that you have to have a decent back surface and know that the glass itself is free of stria and so forth. In addition, you really need to ray trace the lens so that you know what the results should be.

Next is making a test plate and doing the convex measurement just like doing two flats against each otehr. Problem here is that it is easy to scratch the surfaces as well as having an extra surface to do.

Next is using a larger lens to test the surface. Problem here is that you need a big lens to do the test and the test also needs to be raytraced before using it.

Next is the Hindle test. Problem here is that the Parabola is a bit more difficult to test than a hyperbola is. You also need to make a large spherical surface of a fairly short focal length.