Yabia > Who decides who is and isn’t s Gadol?
You do: you ask a shaila of your Rav, and your Rav when he is not sure asks another Rav; people at the top of this shailararchy are the poskim. You are always free to ask a different Rav, or your Rav can ask a different one.
Note that your questions also determine the subject that the poskim are writing about – l’havdil the same way as the topic author in CR determines what the rest are kibittzing about