Rich, Spaced doublet objectives should produce the Newton rings you speak of. It very easy, take the dew shield off the ota. Take it under a bright floresent light (shop light). Look at the refelction of light off the lens. While moving it around look past but into the lens you will more than likely start to see a series of lines floating around in the mix of reflections. Theses are interferance rings AKA Newtons rings. If there in a bulls eye pattern the lens in in good shape, well made and space well... If not then

Of the 10 or so 4" Unitron lenses I have worked on very few were made well enough to be able to produce newton rings. The ones that did were the keepers. The Nippon Kogakus all have perfect sets of newton rings as with many of the small scopes 80mm or less from Japan. What I can say about them is if they are present upon inspection under the floresent light, after colimation the star tests of the lenses with the rings have all been very good. This was true of the Tinsley. The first thing I checked after unscrewing the cover was the newton rings.