The matter becomes complicated when we have a situation where I consider Rav A more qualified than Rav B whereas someone else considers the opposite to be true. It’s often nearly impossible to reconcile such disputes (“my rabbi is bigger than your rabbi”) and we just have to let people act according to their understanding.
Obviously, it is possible to tell who is a bigger gadol! Do you know who is bigger: Rav Moshe Feinstein z”l, or your next-door neighbor? The Chazon Ish, or a local Orthodox rabbi? If a newspaper proclaims someone to be a gadol, and Rav Schach said he was an apikores (for example), hmmm… I don’t know. Maybe the newspaper is right and Rav Schach is wrong? What do you think?