Murder trials, even sensational cases filled with gut-wrenching testimony, usually begin slowly, with a staid jury selection process. The trial of a former FBI agent, R. Lindley DeVecchio, however, should be action-packed from the get-go.

That’s because the first order of business next week will be for the trial judge to decide whether mobster Gregory Scarpa Jr. — whose late father, Gregory Sr., was the exagent’s informer and alleged accomplice in four murders — is a credible witness whose testimony about the long relationship between his father and Mr. DeVecchio is worthy to be heard.