sushe-I’m not sure if your question is sincere or sarcastic but either way there’s no exact science, usually it would have to be by word of mouth from people-more than 1 or 2-with first hand experience dealing with that rov-not from fans who are gushing about him but have never actually worked with him on sensitive situations.
I just had a situation where a rov was rodef me,falsely quoting a gadol against me, he got a second rov involved who later admitted then he went along lishem shamayim without doing any factual research. I contacted rabbonim who knew the story well and who quickly approached the gadol who confirmed that I was in fact the victim in this case. The 2nd rov backed down and apologized admitting his naivete, but much damage was done in the interim and the point remains that one must choose his rov carefully. I know of many stories where a rov either sided with an abusive husband or gave the wife foolish and irresponsible advice. Choose a rov carefully is all I and other posters are saying.