It's not always about the number of houses sold.
Get recommendations from friends, neighbors, fellow employees, etc for starters.
The person that sells the most may not have the demeanor to work with you. You might need a person that does not do a lot of work, but does it well because they can pay more attention to you.
Sometimes, the alleged superstars give some of the work to new underlings who are not as skilled. Also, just because someone has a lot of signs up does not mean that they are the best.