Recently I had a checkride and DPE noticed that one of the Cowling Fasteners is missing Cessna172 (I did not notice it in the preflight). Asked him for time to investigate if safe to fly with one fastener missing; so I called a mechanic and he did not know the official answer. So I asked for a discontinuance. Did I do the right thing, asking for a discontinuance? If I answered, yes it is safe to fly and it is not, then he would have failed me, right? I cannot find the proper answer to how many Cowling Fasteners can be missing and still be safe and airworthy to fly. Where can I find an answer from an official source?