An article and a method for testing the assembly of that article are disclosed. The article includes a plurality of modules, at least two of the modules are capable of being placed in two different positions in the article, each module having a correct position in the article. Each module includes an aperture at a location determined by the desired position for that module in the article. The apertures are placed such that the apertures will be aligned to form a transparent channel when the modules are arranged in a predetermined pattern with respect to one another, but not when arranged in any of the possible incorrect orders. The article can be tested for assembly errors by transmitting a test light signal into the first end and testing for light that traversed all of said first transparent channel.