Put it in a rest or lead sled a long piece of string on the trigger an oiled proof round in the chamber cock take the loose end of string go for a long walk to the end of the length of string then pull to fire reinspect then repeat several times if nothing found. Report findings.
Very tempted... I thought about doing something like that but then I thought... "what if the last test round I fired was the one that weakened it's fault (whatever it is) beyond safe limits". I'd never be sure the next round wasn't going to be my last.