Know how academically successful and respected A professor is
Since there are no answers yet, I'll try to combine the excellent comments into an answer.


First of all, someone speaking highly of themself is a bit suspicious. 
You should read their papers - if you are unable to judge whether they are relevant, check how often they are cited. 
You could check the H-Index, but since most indexes accumulate points overt ime, older persons are having an andvantage


Most probably, it does not make too much of a difference how good the reputation of your professor is as long as you are getting good guidance. Ask former students how good the support was and how interesting the topics were.